Though weighing per wheel( pair) or axle is the best way to determine the loads ,in lack of that you can use the Gross Axle weight rating of your rear axle of your towing truck.
For law you are not allowed to go over that , even when towing a 5th wheeler.
You can mostly find that on the same plate as the pressure advices.
From your tire we already know 2778lbs AT 80 psi for dual so E-load.
Need to know the speedcode .
From TT you gave dual load rear axle so 2 twinwheels on the axle.
But when you are busy also give details of front of TT and 5th wheelers tires and GAWR's and number of axles and total number of tires, Probably ST tires or else give speedcode too.
Then I will do the whole calculation and give advice with maximum reserve with acceptable comfort and gripp and no bumping