Screw your 20% safety margin. If you are looking at RVs near half ton tow ratings where the reduced one is a factor, then it's too much. If it were a boat, then it might matter.
Most half tons will be limited on something other than tow rating/GCWR, substantially below them. Start with your truck's receiver. What is the rating when using a WDH? 1050 pounds? You will want something with a dry tongue weight at least 200 pounds lower than that rating. Next payload, check the Tire and Loading sticker on driver's door or door jam. That has to include people, cargo in the cab and bed, and trailer tongue weight.
For a couple, stick to under 7k dry, under 750 dry TW and you should be pleased with how the Tundra tows (assuming 5.7L/4.30).
