With 1,800 lbs of payload, you're already going to be eating up 700-800 with a family, gas, cargo, etc. So you're looking at a 5th wheel with approx. 1,000 lbs of tongue weight, and those are really tough to come by.

If you're willing to stick with travel trailers, I think the Titan XD would be the better choice of those two with the warranty and better ride.