gmw, you bring up an interesting point, all vehicles have their limits. It also sounds like you have experience towing things. I do believe the manual trans helped your situation.
One thing not mentioned here by the OP, or you, is how many people and how much gear was going in the Frontier cab and bed. Even at 4000lbs, the tongue weight is going to start chewing heavily into the Frontier's payload which is not all that much.
If the OP is planning to pull more than 4000lbs and has wife, 1-2 kids, full tank of fuel - payload is maxxed or exceeded. That is unless mods have been made to the truck.
The Frontier is strong, it'll pull more than its recommended weight. I love mine, just wouldn't put myself on the road with a big box and a full or over weight load in the truck.
I'm not suggesting the OP get a new truck, just a camper that better matches the abilities of the truck and maybe leaves some room for the truck to perform better and have some reserves in power and brakes.
