Easy way to determine how much Travel Trailer I can buy?

Your max towing number is only achieved with an empty truck. If you add passengers and gear in the truck then you will need to reduce the size of the trailer.

Use the gross weight of the trailer because it will be much closer to that weight than the dry weight. Figure tongue weight at 13% of gross trailer weight because again, it will be much closer to that than the listed dry tongue weight.

With 3.42 gears I would limit total weight to 6500 pounds. Even then it will struggle in the mountains.