We bought our tanks from Ronco. They have a great selection of tanks. Its funny that they have so many tanks but it still took a while to choose the right ones. They will make them to order for you but it will cost you a penny or two for that. The tanks are high quality tanks and I don't think I'll ever have to worry about leaks from them. 3/8" walls and the fittings are spun welded and all fixtures are exactly square off the tank. They did a great job at putting the right fixtures exactly where I asked for them. Linda who I spoke with from Ronco (I think it was Linda) got me the tanks for half the listed price.
If you want to do an "L" shaped tank - it might be easier to find (2) rectangular tanks and just do as John mentioned and interconnect them. This also might give you opportunities to change it up down the road as well as make it easier to pull the tanks if you ever had to. I built a "cage" for my Fresh/Gray tanks as neither is strapped down but tightly squeezed into the cages to fit. There is a chance they could have some vertical movement, especially on a bouncy road but so far we have had no issues with setting up the tanks this way. The black tank is strapped in tightly and will not move.
We are fulltiming with 4 kids and have (1) 115 gallon Fresh tank, (1) 115 gallon gray tank and (1) 40 gallon black tank. We can go normal about 4 days with that setup, longer if we stretch it. It can quickly run low in 2 days if we are all showering and do a load of laundry. With the gray tank being the same size as the fresh tank you really only have to monitor your fresh water supply as when your fresh water is out your gray tank is full. Unless you fill up at some point without dumping the gray. I looked at combining the gray/black, having more fresh water than gray and having a larger black tank but after talking to a few other well seasoned fulltiming and RV pros they convinced me that the best system is matched sized fresh/gray and a separate black tank.
One other thing we did was to attach a dual house type water filtering system inline with the fresh water connection. This filters the city water coming into the bus whether it is going to the tanks or straight into the coach. If it goes into the tanks it gets filtered a second time (once when we fill and again when we use it) before it gets used. I also installed a fresh water drinking filter at the sink (If you read our blog you will see that has caused some problems as well...nothing to do with the plumbing though) So we can filter the water a third time with a drinking water filter. Ultimately I'd like to put a ceramic filter at the fresh water drinking tap but we needed it in a hurry so everything was bought from the big box home repair store.
Im amazed that you have 178 gallons of fresh water! Thats a ton of weight!
Let us know what your project is and what you decide and how it works out.
1984 Model 10S