You can estimate 12%.....
But I recommend 15%.
TTs are sort of unique in that the TW can vary dramaticaly in the course of a trip.
Fresh water gets used, and travels to the grey/black tanks. Propane gets used and disappears. Food and drink gets consumed and ends up in the black tank. Clean clothes become dirty and go in the hamper and many other items may get used and end up in different locations that they started the trip in. Some may find that this all balances out about the same... Others not so much.
But nobody here can say for sure what your situation will be. Even someone else with an identical TT may have very different usage patterns than you do.
So plan on 15%, and if it ends up there at some point, you are covered.
I also would recommend looking at 1 ton SRW trucks. They can be had for just slightly more than the 3/4 tons and have much more payload.. In some years/models these may be called HD 3/4 tons. Same truck, different label.