There used to be a company that sold a kit to put a spare tank where the spare goes, I think thats the best you'll be able to get, dont remember how many gallons it was something like 10 or 15.
Dont remember the manufacturer either but they were sold through J.C. Whitney.
A friend of mine uses an auxillary bed tank in his short box chevy, I think it holds around 75gl, it was originally designed for diesel to dispense to equipment in the field but he modded it a bit and plumbed it into his fuel system.