I'm towing a 2502 with my Xterra, which is limited to 5000 lbs. Tows it fine, I just leave the transmission out of O/D. Puts me in 4th gear ~2500-2750 RPMs.
I would have concerns about an Ultralite too. However, my buddy has owned is Rockwood TT since 04, and has traveled from FL up to Canada -n- back the first year, then from FL to Oregon -n- back the next. They've been fulltiming many months each year. Their camper still looks new. I was so impressed, I ordered a smaller version. Don't let the Ultralite label fool you, it's well made.
Here's a picture of my frame:
Here's a picture from inside the storage bay. Note the aluminum framework that supports the bed above:
Aluminum framework is also used in the dinette frame, and in the walls. Very strong yet lightweight.
There's no noticible flexing of the floor.
Can't tell you what all they did to save weight, but they did use Filon walls for the exterior, and the roof is a few inches of foam with about a 1/4" luan skin on it. Strong enough to walk on, yet saving a lot of weight.
I would buy one again without hesitation. Good luck on your quest!