I am just posting my observations here, I have owned 4 diesel trucks, and multi gas trucks, but only hauled the TC on the diesel.
for what I see on average, most newer gas trucks get a bit better MPG than they did lets say 10-15 years ago, so I only see maybe 2-4 mpg difference in diesel and gas while hauling the TC, My opinion would be that a newer gas truck lets say 2008 and up with the biggest power plant option would be a fine truck for a 2,500-3,000lb camper.
and another note, we have many shop trucks where I work that pull 18ft job trailers loaded way over there rating, we have chevy and ford, the highest mileage chevy is an 08 and has 281k still runs like a top, highest mileage ford is a 2010 with 244k, also runs like a champ.
all trucks are 3/4 ton.
the ford seems to have a bit nicer interior.
I would think for less than 10k you could get a pretty nice gas 3/4 or 1 ton.