I'm actually in the same situation as you, except I don't own a car. I've been looking for about six months now so I'll fill everyone in on what I've found. I'm very concerned about mpg as gas is a major cost in racing. One thing I've learned is that America is against high mpg vehicles.
All mpg figures are highway.
Trailer - If you already have a car and you can easily store a trailer, then you should definitely get one. An enclosed trailer will prevent theft as well as damage from the elements. All cars can tow, just look at the european ratings for the limits. If I had a place to store a trailer I would be pulling it with a VW TDI. You'll have to be concious of the weight, there are very few lightweight aluminum trailers. Remember the 55mph speed limit and the extra axle on toll roads. It's also a hassle parking and backing up.
Minivans - I don't think of minivans as an option because their mpg is terrible...low 20s, with very limited cargo space. That's not worth it at all. For such a small van I expect nothing less than 30mpg+, but only the newest models do that. Plus loading/unloading a minivan takes twice as long as any other vehicle. Windscreen has to be removed or fairings taken off. Dirt/supermoto bikes require a small front wheel in order to fit. Anchoring the front down to make the bike fit is bad for your fork seals.
Sprinter 2000-2006 - Upfront cost ~$15k...the high roof only gets ~23mpg and maintenance costs are high. Low roof models can get ~25mpg+. 2007+ are all low 20s. The lwb version is ~23ft+ long and needs two parking spaces, but 14ft cargo length. the mwb version can be parked in most spaces at 19ft with 11ft cargo length.
Astro/Safari - An Astro/Safari is a good compromise for a smaller van that's bigger than a minivan. Low 20s mpg and decent cargo space. I had a similar size in Japan and could fit two bikes, but had to place my gear all around the bikes, scratching up the fairings... Plus you have to wait for the exhaust to cool or you'll burn something. There are SWB, normal WB, and extended versions. 1985-1994 get better mpg.
GMC/Chevy 1985-1994 - A longer van with low 20s mpg is a 1985-1994 v6, conversion, Starcraft, Vandura, G1500/G2500, G10,G20. Most are V8 with < 17mpg, but if you can find athe v6, you're in luck. There are also some high roof models, which are awesome, but it's rare to find a v6. The cargo area is a little longer than an Astro, but the increased height is worth it. Certain years get better mpg, late 80s get 1-2mpg better than early 90s.
Toyota Van 1983-1988 - 80s Toyota vans get the highest mpg at 25+, but the rear door opening is only 4ft tall and there's 7ft to the rear door behind the seats. It feels cramped inside and pretty narrow, like a minivan.
Dodge/Ford - All dodge/ford vans get terrible gas mileage, < 17mpg.
GMC/Chevy 1995+ - I've seen some get 20, but mostly < 18mpg.
VW Eurovans - are nice with the poptop someone just showed, but only 20mpg and the poptop versions are way overpriced. Plus it's common for the transmission to need to be rebuilt every so often.
VW Vanagon/Bus < 1992 - Not an option, they have an engine in the rear so there's a huge hump, plus the rear door is maybe 3ft tall.
Vintage van - A really cool option would be a vintage ford/chevy van from the 1960s, they get 20mpg but you'd have to do some custom work to the transmission to be able to cruise easily at 65mph. Normally you're looking at 55mph.
Trucks - Terrible mpg, bike/gear susceptible to theft and the elements. Cargo space is too small. Trucks are are only good at hauling large trailers.
Step/Box van - ~10mpg
I think I covered everything, I hope that's not too much info!
