I'm glad to hear you like your trailer too!
First let me appologize if this post gets long, detailing technical info can become wordy!!
Nissans tow rating of 5000lbs for the Xterra is a bit strange to me. The Pathfinder has a tow rating of 6000lbs, the Frontier has a rating of 6,500lbs, but they all share the same Box Steel Frame construction, same engine, same transmission (they do have slightly different gear ratios, they use the same torque converter, clutch pack, valve body, etc), same size brakes, same size radiator and all have very simular curb weights. When you look at the fact the older Xterra's had a 5000lb rating, and it had a smaller transmission, 95 less hp, 85 less torque, it seems odd this new, stronger, more capable truck retained the same rating. I believe it was more of a marketing decision, have the cheaper Xterra retain the 5000 rating, and have the more expensive Pathfinder have a higher rating. Now I can't get Nissan to admit that, but the service people at my Nissan Dealer have told me the rating on the Xterra is conservative. In my opinion, the Xterra should be rated at either 5500lbs, or 6000lbs, but that is my opinion.
Our first trailer was a Coachman Spirit of America 24RBS, which we bought off my Father-in-Law. The 24RBS had a sticker weight of 4680 empty, add in dealer options, full propane, fully stocked with food, clothes, chairs etc and loaded for camping it tipped the scales at 5400lbs, without water in the tanks and the Xterra towed that a couple thousand miles. I didn't like the layout of the Coachman and the plan initially was to get a larger truck, but first we thought we would look to see what ultra lite trailers were avail. When we saw the 230DS it was a all new model, just released from Cruiser and the sticker weight showed 4188lbs. My wife loved the layout, so we decided to grab it and keep the Xterra.
We used the Xterra to tow the Coachman a few times and it handled it without issue. When we began towing the FF it felt pretty much the same, except our mileage increased and steep hills were a bit easier. When I weighed our loaded (No water in tanks) 230DS it weighed 4720lbs. It is lighter than the old unit, but up near the limit of the truck. The combined weight of truck, trailer, supplies, my wife and I and our dogs is just at the 9650 rating of the Xterra, but again, in the 7000 miles we have logged towing we have not had an issue.
I do want to add that when I tow I use common sense and I don't push the truck hard. I keep the speed to around 65mph, I don't push the engine past 4500 rpms when towing, I use lower gears coming down hills, and I always use a weight distribution hitch to spread the tongue weight between the truck and trailer. I also perform vehicle service sooner than than Nissan's schedule says to. At 10K miles I drained and filled the trans fluid, then again at 20K, and did a complete flush and fill of the trans, radiator, rear diff, steering fluid and brake fluid at 30K.
I feel very comfortable towing our trailer with the X, it stays with traffic and performs without drama, it is a good match to our FF.
Your two Nissans are very capable trucks, they will handle any trailer in the FF lineup without problems, so go get a bigger one!!!