is it possible to add a second fuel tank

Absolutely it can be done. My Toyota has a 2nd take with a pump to fill up the 1st tank. Some people just opt for 1 bigger tank. IT's 12V and controlled with a switch. When fuel shows low, flip the switch on the road and 30 mins later it shows full again. :)

When doing your calculations figure out if you have enough CCC to handle the extra weight. one 36 gal tank sounded like a awfully small capacity. Have you gotten under it and looked you might have a second tank someplace.
Bill

Would the 55 gallon fit behind the rear axle? I wonder if it's deeper or longer, would same fuel pump/sending unit be used?

A 55 gallon tank from a newer e series would probably for fine. As would the 35 gallon midship tank.

well Fleetman, transflow site only showed options for F series fords, no E series. I may measure area between drive shaft and frame, then go to salvage yard and look for a van with mid tank, and see about getting tank and mounting parts. then add an electric fuel pump and hose to transfer fuel to existing tank.

A good metal fabricator can to any thing. If you were near Orlando, I could send you to two of them. not cheep around 500 or 600 for a custom built tank of 1/8th inch aluminum, plus the install. This would last forever. and be whatever size you needed.

There are several companies that specialize in bigger tanks and additional tanks. TransferFlow in California is one. I replaced the small tank in my F-250 shortbed with a 48 gallon tank from TransferFlow.

A friend replaced his 36 gallon tank with a 75 gallon after market tank. Was awhile back goggle replace tanks for Ford vans!

West,

You would do best to ask that in a Fleetwood or Ford group. I would bet that someone has at least investigated the possibility. There are Fleetwood groups here, on IRV2 and Facebook.

Matt