You refer to this cell/tank in the Singular, so I take it that you don't have an additional Side Tank. I actually thought those had gone away by 1988, but that's what our 1983 E350 had. Two tanks. RockAuto.com shows a 22-gallon rear tank and a 16-gallon side tank for 1988. Both priced well under $100 each.
Do you know the capacity you have now? Newer E350's used in Class C's (by that I mean 1992 and newer) have a 35-gallon tank with 55-gallon optional.
Anyhow, RockAuto furnishes the dimensions of both those tanks. You might be able to install two, but the cost goes way up since the Selector Valve is about $100 and you'd need lines, wiring, a switch, relay and ....
What about a later model 35 or 55 tank? You don't need to just know the dimensions of the early and late model tanks. You need to see if Winne added components that limit the space for the tank(s). Remember too, correct pump for your engine may not fit a different tank.
If you have an onboard generator, you have to provide a fuel connection for it.
Finally, how do you plan to use the RV? Even if you plan to go cross country, there's gas within 100 miles almost everywhere. Alaska Highway off-season, maybe not.
If God's Your Co-Pilot Move Over, jd
2003 Jayco Escapade 31A on 2002 Ford E450 V10 4R100 218" WB