So, here's how I approach things (and just so happened to make calculators for it :)).
1) Find out your ballpark numbers- you'd be amazed at how much payload gets eaten up:
Towing Calculator based on Truck's Payload(it's even better to load up like you're going camping and actually get weighed- (
calculator)
2) Then, to get a better feeling if 10% or 15% of tongue weight is applicable in your case, you can use this estimator to take the reported dry weights and extrapolate the tongue weight percentage. You can then either take the dry weight + 1,000 or 1,500 pounds (given the size of camper) OR take the GVWR for the most conservative route. And figure an estimate for the actual loaded tongue weight:
Tongue Weight Calculated from Dry Weights