3/4 ton for a 3500 pound GVWR TT? I love mine, but it's overkill.
If this is a low walled RV, then buy a minivan. I like the 05-07 Chrysler ones. Lots available, fairly cheap.
Anything high walled, for $12k age and capability means pretty much a half ton, maybe a mid-sized SUV/truck. But fuel economy towing is all the same, as is unloaded for that vintage. My 2003 F-150 used about 15% more fuel than my 2011 3/4 ton Suburban, but this Suburban was not $12k!
GM full sized van 3/4-1-ton. 6.0L/6-spd/3.42 axle. Dirt cheap used. Very capable. And decent fuel economy.