Hmm- IMO only way to get that kind of mileage and be able to tow is a diesel- ideally turbo diesel-
SO- options- get an old UPS/Fedex diesel truck- ditch current body/shell and add something more aerodynamic (or just "chop" it!!). Ideally the vehicle will be aerodynamic and essentially break the wind for your cat- also get a "narrow" cat- Taipan, Wave, "A" cat- a lightweight trailer with tall, narrow tires, mount the mast horizontally with mast stands made from old mast sections (at least up where they catch any wind- or slit some old mast sections and mount over what you have). Ensure you have a good highway rearend (ie tall gear ratio) and a manual transmission. Have a small passenger compartment (2 people) with added insulation in roof/floor/doors and use a small AC compressor.
Another option is an old Ford diesel Ranger pickup, Isuzu diesel or VW diesel pickup or old diesel Jetta wagon-

Kirt