You mentioned a Grand Cherokee. Do you own one?
If so, use it.
Bigger trans cooler, maybe some Air-Lift bags and see how it does for a while.
Another option to consider is an Expedition. A buddy just bought one to tow his bread trailer (distributor) and loves it.
20mpg with anything that will tow that trailer easily isn't happening on your budget, and would be hard to find with a much larger budget.
My Tundra gets about 16.5 mpg empty, but it's rarely empty.
A trailer like yours, loaded and it's going to be around 10-11 mpg probably.
Any diesel you find at or under $6K I wouldn't want. Folks here treat them as if they are made of gold, prices are stupid.
I know a guy with one of the GM military trucks, and a Blazer (CUCV?) he swears by them, but he is an old fart and I doubt he's been more than 100 miles from home for 20 years.