I'd recommend your favorite flavor of 3/4 or SRW 1 ton for that size trailer, gas or diesel. Gas would certainly be adequate but diesels perform quite a bit better, so that's just a preference thing - either would do fine at that size/weight.
That's a bit more trailer than I'd want to tow very much with a half ton, both with respect to weight and length.
2015 Cummins Ram 4wd CC/SB