The bunkhouse requirement suggests children or multiple adults going on camping trips. That means a reduction in available payload and towing capacity. They may not have room for that 900 lb tongue weight, or 9000 lb trailer weight.
Here's a link to a calculator they can use to get a ball park figure for loaded trailer weight that they do have room for.
Things they should keep in mind:
1. Trailer sales people talk in unloaded weights (UVW) for both trailer weight and tongue weight. On average, loaded trailer weight is about 1000 lbs more than UVW.
2. Average tongue weight is 12 - 13 percent of loaded trailer weight. At 12.5 percent, a 7000 lb trailer would be pushing that 900 lb tongue weight limit.
3. Tongue weight is not a constant number. It fluctuates (sometimes up to 200 lbs), up and down, during every trip.
4. There's a tire / loading sticker on the drivers door (or door post) that shows a "max occupant / cargo capacity" on their specific Tundra (they're not all the same). That is that truck's capacity to carry the total weight of any aftermarket accessories that may have been added to the truck, passengers, cargo (in or on the truck), a weight distributing hitch, and the trailer tongue weight.
5. Being on the edge of the truck's capacities, can make towing an unpleasant experience.
You mentioned a bigger truck could be a possibility, your friends should find out what they have room for, before they go looking at trailers. They may find their Tundra doesn't have the capacity to tow the trailers they are interested in.
