Over the years, there have been a number of discussions about what to used for backer boards on tiled floors and walls. Usually the recommendation is to get a 1/2-inch cement board such as "Wonder board" or Hardi-Backer. I once recommended a Georgia-Pacific product called "DensShield".
There are some web sites on the internet panning DensShield because "it contains gypsum" and therefore must be inferior to cement boards because we all know that when gypsum absorbs water which would happen if the waterproof/water-resistant acrylic finish is penetrated or broken, it also disintegrates and loses its strength. People who say such things are uninformed about the product and are publicly displaying both their bias and their ignorance. Experienced tile setters tend to resist the product because "what they've been using works" and they don't want to "risk" using something different.
One tile setter told me he threw a piece of Hardi-Backer in a container of water and left it for months with no discernable deterioration.
FACT: There is a BIG difference between something that doesn't come apart in water (like normal gypsum drywall board does) and something that won't allow water to get into your structure and do bad things to it.
To properly install cement board, you !!!MUST!!! put a waterproof membrane behind it but ahead of the framing so that any water that gets that far into the wall cannot possible get into the framework. With DensShield, you DO NOT USE A WATERPROOF MEMBRANE behind the backer panel.
Here's the straight stuff on DensShield:
DensShield uses a core material made up from a mixture of gypsum, vermiculite, silicone binder, fiberglass reinforcing strands (similar to "re-mesh" in ready-mix concrete commonly used by concrete contractors), and other agents. The panels are covered with a fiberglass mat on both sides and over the long edges of the panel. The tile side of the panel is coated with a baked-on acrylic finish that has a better adhesion to thinset mortar and tile mastics than WonderBoard, Hardi-Backer, and other cement boards. DensShield is also lighter in weight than concrete boards, and easier to work with (just use a normal utility knife like when working on normal drywall installations).
I am currently remodelling a bathroom in my home, and have been using DensShield around the tub/shower and on the floor where I am installing ceramic wall tiles and porcelain floor tiles. The floor is 1/2" plywood installed when the house was built in 1976, with 5/8" OSB on top instead of the old particle board I ripped out. I bonded the two together with parquet floor adhesive I got at Lowe's ($10 cheaper per gallon than a competing product at Home Depot). I then nailed both to the joists with 16-penny nails.
I then installed 1/4" DensShield over that by laying a layer of latex-modified Portland cement thinset mortar using a 1/4" x 1/4" x1/4" square-notched trowel, placing the 4 x 4-foot panels on the OSB and screwing it down using epoxy-coated Phillips-head flat-head screws (1-3/8" long). For walls, the 1/2" x 4 x 8-foot panels of DensShield are attached to studs on 16" centers (or less in some places, but not more than 16" anywhere per manufacturer's directions) using the same screws. Wall panel screws are on 6" maximum spacing and the floor screws are no more than 8" apart on an X-Y grid. Panels fit tightly together and the mortar tends to squeeze into the narrow space between panels. Before installing tile, lay 2" fiberglass mesh self-adhesive drywall tape (10 strands per inch spacing or closer) over the joints, then apply the thinset across the tape.
The same latex-portland mortar is used for installing ceramic or porcelain floor tile (you need the high-flex kind. Some have epoxy or other polymers as well for superior adhesion. Again, use the 1/4" x 1/4" x 1/4" square-notched trowel for 8" or larger floor tiles. It's also suitable for 6" tiles, but for smaller tiles, you may need a different applicator. For wall tiles, use ANSI "Type 1" adhesive and non-sanded grout. Sanded grout is good on floors, and a polymer/latex additive for flexibility may be an advantage in buses due to vibration.
I am not done yet with the tile work, but so far, I am very pleased with the behavior of mortar and thinset with the tiles I do have installed.
DOES IT WORK?
A test lab assembled a normal shower wall using DensShield with normal wall tiles, but no grout was installed between the tiles. They then sprayed the wall with 110-degree (F) water 15 minutes (with 45-minute gaps) once each hour, 24 hours per day, five days per week, for a full year. At the end of one year, there was no moisture penetration through the wall, and the tile were still fully attached with no loss of adhesion.
An identical test on "green board" lasted 10 days.
Georgia Pacific also makes a replacement product for "green board" that is now stocked at Home Depot (at least here -- not sure about all stores). It is a gypsum wallboard with fiberglass in the core, and the outside is coated with fiberglass, but it does not have the waterproof acrylic tile-mounting surface of DensShield. It is also a very nice product for use in areas where you don't hit it with water, yet the humidity is high much of the time (but not all the time). I think it's called "DensGuard" or something like that. It has no paper or plant fiber in it, so it doesn't support mold growth or anything else. Joints are finished using the same fiberglass drywall tape and a setting-type drywall joint cement. That is the dry cement you have to mix with water and it hardens in a specified time: 20, 30, or 45 minutes, depending on the version you get. Once the joints are bonded using the setting-type cement, you can then use normal pre-mixed joint cement or topping compounds to finish the job if you prefer.
The "real" way to test backer board is not to toss a piece in water for a few months. Instead, take a piece of ABS or PVC drain pipe (3" works well) about a foot or so in length. Set it on end on top of a horizontal piece of backer board you want to test. Lay a bead of silicone caulk around the outside bottom of the tube to fasten it to the backer board and also seal the joint at the same time. When the caulk has hardened, fill the tube with water and wait to see how long before water drips out of the back side. This is the water that will get into your wall framing if water gets past the tile grout into the gaps in adhesive behind the tiles, then builds up until it finds a way through the thinset and reaches the backer panel. With cement board, the water will wick into the wall interior. With DensShield, it can't get through if you follow the manufacturer's instructions.
Residential tile installations on DensShield when instructions are followed are guaranteed to not fail. Motorhomes may not carry the same warranty, but I can clearly tell this stuff will stand up as well as any cement board out there.
I like this stuff...
Just remember that tile backer must be properly installed so that it is not expected to carry the forces that are supposed to be carried by subflooring, framing, etc. If the backer flexes in use, that can be bad for grout, and if the grout fails, there may be some risk on tile-setting adhesives. But with a full-size bus and proper structure underneath, the tile should not be a problem when installed over DensShield.