I have had a few Toyota trucks, all very dependable, all easily maintained and rugged, but none of them got better than 21 mpg and you could get out and run faster than they could drive up a mountain pass.
Currently, I have a Ford Ranger, which has a v6 that gets 24 mpg and will go as fast as I want up a hill, (better gearing ratio I think), but I can tell it won't have the lifespan my toyotas had, (230,000 miles and sold them as daily drivers).
My dad has a ford powerstroke for work and a dodge cummins for play- both are high mile rigs;the cummins gets 26 mpg with a big camper in the bed, however, things you have to consider with a big diesel:
as someone mentioned earlier,
-oil changes cost $75+
-Tires are 16 inch or larger, (now the same with most vehicles) and usually have to be load range D or E rated- costing $850/ set.
-Diesel engines are heavy and front ends get beaten up by the weight
having said this, there are theories that a bigger engine is more efficient than a vehicle with a slightly undersized powertrain that has to constantly work hard to compensate
If you do choose a diesel truck, look into propane injection over veggie oil, it improves fuel economy by 4-6 mpg.
The vehicle I'm currently examining is an AWD Astro or Safari van- had one as a kid, 26 mpg and built on a truck chasis, so its a little more rugged than a typical mini van and can be lifted if you realyneed the clearance. check out astrosafari.com;
I'm searching for a long wheel based cargo in AWD. check em out