Anyone suggesting that you use that water heater for radiant heat has their tongue in their cheek. That device is SOOOOO inefficient. The hole in the lower right is the end of a "U" shaped pipe that exits the tank at the upper left behind that grill. That exhaust gets so hot your skin will "stick" to it if touched. The water is heated by the u shaped "chimney" that is in contact with the water inside the tank. After the water is heated the flame goes out but that chimney still keeps ambient air flowing thru the chimney and cooling the water. It is a very inefficient device but it works very well and there is one on every coach I have ever seen unless the have an electric HWH.
It can be made much more efficient by putting an insulating pad over the entire outside assemble and securing it to the skin. Insulated and closed up that way you must use an electric element to heat the water. I have full timed with this and it was no problem....used a lot of propane when elect wasn't available.
There is a "on demand" system that heats only the water you are using and quits when you turn off the hot water. They have them with propane and electric. Electric one are wimpy and voracious power eaters. The propane ones cost $700 but aren't as expensive as it seems considering that one you have sells for a few hundred. The on-demand is also much more efficient even though it sounds like a jet engine when fired up.
While you are looking at all that propane stuff make sure you don't have any compression fittings in the lines..only at the device and the manifold...each end. The copper tubing should run from the manifold to the device and have no splices. I took out all my long and cumberson propane lines and spent a day carefully cutting the tubing and fitting it really pretty with compression fittings. Right angles around wall hugging corners and perfect verticals and horizontals.....and what an idiot I was. Each and every joint on that line was a "failure point" and I installed about 20 of em. Propane usually only leaks once and that is the end of the coach....and you if you are in there. I redid everything I had done previously and took it back to original.
Just thought I would jaw awhile seeing as you said you didn't know what was in there and were a little green.
It is my understanding that propane uses ONLY 45 degree flare fittings, never compression.