Good luck there dude.
Couple of points of reference: my 97 F150 with the big v-8 would get low 20s on road trips.
The F250 with the diesel would too, but that truck was geared to go 55mph.
Around town? Both 14-17mpg
I'd look into the MB 'SUVs' with the diesel for towing something light AND getting good mileage doing it. The Tuareg with the diesel isn't designed for economy.
Can you rent a van for those 'pack it up and haul it' trips and keep a diesel car for daily use? Save in the long run despite the PITA of picking it up and dropping it off.