Most of the options have been stated already.
Depends what you want to spend and/or how old of vehicle you're comfortable with.
New, Burb 2500 or 1 ton van are about the only 2 I can think of. WIth the van you may be able to get a diesel, but no 4wd unless you get a very spendy conversion, like Quigley.
Older vehicles you have the option of the Burbs with even a 8.1 big block, or the Excursion.
We've made a couple 500mi round trip road trips in a 05 Expedition, 4 adults, 4 kids (1 car seat and 2 boosters) towing a 5500lb wakeboard boat and the Expe handled it much better than I would have expected, but it was not ideal.
Personally, look for a 2500 Burb or a cherry 7.3 Excursion diesel and make it last.