I have a 35 gallon in the bed and use a Mr. Gasket Micron Electronic Fuel Pump to transfer fuel to main tank. Been using one for 4 years not the fastest but works for me. About $40 at O'Reilly Auto Parts. Be sure toi get the one for the 8 cylinder engine.
1999 3500 Red dually pulling a 2005 36ft Hitchhiker II,, Now stock with straight exhaust will install Edge EZ next week.