If you don't have an enclosed underside, get under there and see if there is any room. If there is, measure and see what might fit.
There are certainly bigger tanks, but finding the right one and actually installing it is a little more challenging.
Personally, I would just go buy a "honey wagon". Will be cheaper and easier and let's you empty either tank.
2013 Winnebago 2301BH-Red
2012 Ram 2500 Megacab HO CTD