It is easy to spend anywhere from $4,000 to $8,000 on a single diesel repair job. For a travel trailer a truck with a gas engine and 4.10 gears are all that is needed. It will be cheaper to buy and cheaper to maintain and cheaper to repair and cheaper to fuel.
Check the Ford, RAM, and Chevy diesel forums to get an idea of the problems that can arise and the cost of the repairs. Talk to people at the local diesel repair shop about a head gasket repair or replacing the fuel rails or a compressor.
Any used heavy duty truck is quite likely to also need front end work with new tie-rods and ball joints before you take it out on the highway. They are supposed to get greased every 3,000 miles but doubt that one in ten get it.
Lots to go wrong with diesel pickups besides the "engine" and gas engines are a lot more reliable as they are a lot less complicated (no compressor or high pressure fuel system or exhaust brake).
Check around town and see what percentage of the gas stations pump diesel. I have to drive 20 miles round trip from my house to get to one that pumps diesel. With gas the distance would be 3 miles and I would have 3 stations from which to choose.