www.acmeadapters.com
for a VW turbo-diesel conversion kit so you can do highway speeds with the weight.
Or perhaps a 1.6 tracker engine would do ya'.
My 1.9td Samurai would get 27 mpg at 65mph pushing a lifted Zuk w/35" tires - can't be much better (if any) than your camper for wind resistance.
Mind you, the diesel is very torqey, and tough on the trans, where a 1.6 16valve Sidekick swap will make power with rpms, so you can put less torque to the gearbox to make the same power (with gearing).
I like your camper - I'm thinking of something like it so I can make my zuk into a snowcat, I wann' run the Alaska Highway in winter someday (it's been done by a guy in a Jeep on a set of AmericanTruck Tracks- everything has been done
)