i was really thinking about a fuel cell. but i didn't want a gas tank under the bed. so at the risk of being completely origional, i'm installing a diamond plate combo tool box/tank. it is pretty normal for people with 5th wheels to have an aux tank like this. but i really like having the combination and having a 50 gallon capacity. looks attractive, holds alot, strong as hell, easy to get to (no fabbing fuel doors) check out
www.transferflow.com just for kicks.
just my $.02