I have had 2 rv's with aux tanks. both class C . One was similar shape to the stock tank, behind it. (MB300 dodge, chassis, both of them) The other was a thick gauge rectangular outside the frame with the fill made of galvi. pipe at the camper wall surface immediately behind the cab. I removed that one because it was outside the frame and we were concerned about safety. At one point JC whitney offered aux tanks for vans and that might be a place to get one. There are custom steel tanks that can be made up. The fill is tricky and the venting is also but it can be done nicely