Happy medium between comfort and good gas mileage to haul?

I believe ( someone can correct me if I am wrong on this ) that some national parks do not allow camping in "soft-sided" ( tents, pop-up with canvas sides, etc ) campers.

I pull a funfinder 189FDS ( just under 19' total length ) with a Nissan Frontier ( V6, 4WD, six speed manual trans ) and get between 10 and 13 mpg. My overall average while pulling has been right at 12.5 mpg.
This truck pulls this 3900 pound travel trailer fine.

When not towing, the truck gets 19 to 22 mpg for normal use.