Re: WTB; Ford E250 or E350 w/penthouse top
Not sure if renting one from Enterprise will give you an accurate figure since they only have empty cargo vans. Once you start adding things, that will obviously affect gas mileage.
Theres a 97 in the OC craigslist that is pretty much a blank slate if thats what you're looking for, except it does have a penthouse top with bed and a cabinet with sink, fridge, and microwave (I think). It also has an outdoor shower and its fully insulated. Looks very clean. It doesn't meet your year specs, but it does have a 5.4 in it.
2005 E350 Chateau - V10 - Agile Offroad 4x4
2012 CTS-V Wagon - For the baby...