Bigfoot trailers have a sticker in the closet that states the weight
with options when the trailer leaves the factory. The contents of the tanks are also included. This takes some of the guesswork out of estimating the weight
of the trailer you are considering for purchase.
Some of the Bigfoot 21 footers have balance/sway issues. One fix is to add a ballast weight to the tongue, which the dealer can do, if not already installed. The fresh water tanks are located in the front or rear, depending on the floor plan and it makes a substantial difference how much water you carry. IOW, the balance/sway can be improved by carrying more or less water in the fresh water tank, depending on the floor plan. I think the fresh water tank is in the rear of the rear bed model and that is the one with more tendency to sway. If you are considering a 21 footer, learn more about this!
The 21 footer is wider than the 17 too. You need good auxillary towing mirrors for the 8'-4" width. The 4.7L V-8 should be adequate for the 21, but you can never have too much power when towing a trailer up a steep grade. BTW, our 17 footer tows so well I don't usually bother with sway control. You might need a Reese dual cam or Equal-i-zer for the 21.
And if you have to go to that much trouble, why not get a 25 footer and a 5.7L Tundra ;-)