I was wrong about the width of the coach body having anything to do with it; the tank is inside the frame rail.
I don't see how the coachbuilders would have been likely to have done anything to affect whether a larger tank would fit and how big the tank could be. I think a tank that fits a Sunrader of a given model year (or set of years) would fit an Odyssey, Dolphin, Travelmaster, Huntsman, Winnie, or any other Toyota motorhome of the same year (or set of years).
BTW and OT:
The seatbelts turned out great. It took me a little while to find someone who would add them. They were done by Airport Valley Vinyl of Tukwila, WA (near Seattle). It was Poulsbo RV in Kent, WA who suggested I ask them.
We added seatbelts to each of the captain's chairs, and to each end of the sofabed (facing forward and facing rear -- for baby seats, not for adults). Now we have three places we can put our baby, who still needs to face toward the rear, instead of just one. He likes to be near my wife, but she doesn't like to drive. Until now, he would fuss when I drove, even if Barbra sat on the front of the sofa, leaned in toward him, and talked to him. Now they can be in the captain's chairs facing each other, or he can be on the front end of the sofa, facing her in the middle of the sofa. When he's big enough to face toward the front, we'll have two additional places we can put him. I anticipate some happier travelling. Knowing my wife, she'll even be able to nurse him like that (she's amazing in that regard).
Back on topic:
Again, we'll be gone for a few weeks, so I'll check back in on this thread when I get back..
Greg