I've been doing thinking about the options here, and there is no good solution. In the end, a truck is probably my best bet. Or an SUV and limit myself to a hitch carrier. At least that'll keep the gear safer.
Own a high mpg little car and tow a trailer:
great mileage when not towing
pretty good mileage while towing
stuck at 55mph in a lot of places
trailer registration and upkeep
not fun to get to offroad riding areas, if the car will even get there.
Own a truck
so-so mileage towing
can get just about anywhere you want to haul the dirtbike
so-so mileage even when not towing
Own both a little car and a truck:
pay for and maintain two vehicles, find parking, insurance, etc.