It's the same for a fifth wheel...the tongue weight of the towed boat trailer will reduce the pin weight of the fifth wheel, that is a 500 lb tongue weight takes 500 lbs off the pin weight. It could be more or less depending how centered the axles are to the front/back of the camper. You can compensate by adding weight in the front (fresh water tank?) if the truck can handle the extra load. If pin weight gets too low, handling may be compromised.
I'm removing the spare tire and carrier off the rear bumper of ours and replacing it with a 2 bike carrier. The spare tire will be relocated in the front compartment. The tradeoff in weight is 20 more lbs on the rear.
Nights camped in 2013 - 55, 2014 - 105, 2015 - 63