For $15K it will probably have to be a gas engine truck. You won't find a taken care of diesel truck for that price.
A 3/4 ton (2500) is ideal. A 1/2 ton will work. Air bags (on either) really help.
I lucked out and scored a very, very clean Silverado2500HD with a gas engine while shopping for a truck for my Grandby. Mine is a short bed but, I drop the tail gate and the back of the camper is sitting on the tail gate. Very hard to find a long bed.
You can't even tell the camper is on the truck. I average 11-16 MPG. 11MPG When I pull a trailer with 2000 pounds of ATV/UV on it.
If your a GM person, keep your eye open for any 06-07, 2500 with the 6.0L gas engine. RPO# LQ4
No AFM (Active Fuel Management) and no VCT (Variable Cam Timing) Its one of the best "LS" series truck engines you can get.
Saw one on Craigslist just recently, 80K miles, extended cab, long bed, 4X4. LT trim. $16K