You're right in doubting the salesman. The 9200lb rating is based on an EMPTY Expedition. No passengers. Nothing. Just you and the truck.
Over the years, I have seen many rigs come and go. It seems like 7000lbs is about the practical limit for that class of truck (i.e. Expedition, F150, 1500-series GMs and Rams), when there is a family in the truck with you (eating into the truck's payload capacity).
That's under 7000lbs, fully loaded and ready to camp. Full water. Food for the trip. Clothes, bedding, toys, bikes, etc..
I would personally look for a trailer with a dry weight at around 5500lbs.
Even at 7000lbs you're going to start wishing you had more truck after a while. It won't be unsafe, and not necessarily uncomfortable. You'll just be wishing you could accelerate faster, maintain speed better up hills, maybe get rid of the slightly mushy feeling that accompanies towing a trailer near the limits.
