Price point will be the major driving factor. A full-size van will be the cheapest, since they're not very sought after in the consumer market. A large SUV will be more expensive, but probably have more creature comforts and higher tow capacity due to it's size and weight.
The tow capacity will dictate engine/drivetrain needs, which is going to affect mileage. 2wd is going to get better mileage than either AWD or 4wd, the latter are also going to have higher wear and tear and maintenance costs.
The 03 Suburban I bought last February is the 1/2ton, 2wd, with the 5.3. In town it gets 12 to 14mpg, on the freeway she gets 17 all day long. I recently towed a utility trailer to the dump, the tipping weight of the load was 2100 pounds, the empty weight of the trailer is probably around 1000 or 1300 pounds for a combined total of 3400-ish pounds and I really couldn't even tell the trailer was there, the only real sign of the load was slightly increased increased braking effort and distance. I only towed this for a grand total of maybe 20 miles, so I can't report any mileage stats for towing.
I found nadaguides.com helpful to know the factory stats on potential vehicle choices. Here you will find curb weight, tow rating, factory amenities, etc. If buying used, it also gives you a value for whatever you spec out, which means you can put the specs of any potential purchase into the calculator to know what it's actual street value is to know if the seller is smoking crack or actually knows the value of the vehicle.
https://www.nadaguides.com/Cars/Manufacturers
Kelly Bluebook is another valuation site, but doesn't have the depth than NADA does.
I've got several other sites where you can check the vin # for recalls, stolen vehicle, general service issues with the make/model you're choosing, etc., if you're interested.
