As has been posted, but look at dry weights and determine if that's within your rating, then demand before purchase allowing you to have it weighed with your truck. Now that if it is within the ratings you will have to calculate for any nessasary liquids, fresh water, fuel (toy hauler) and propane ... ... ... The dealer will not allow you to fill the tanks.
A buddy of mine hauls a triple axle toy hauler with a 02 2500 4X4 with air bags, and does just fine. He fills his tanks with only the essentials and has been doing this for a couple of years now. Sort of like what pilots do when flying at max weights, they will calculate how much fuel they can fill their tanks with to stay within the allowed weights. The problem will be the location of the tanks in relation to where the axles are. For example my AF 5ver has the fresh water tank forward of the axles which adds a lot more pin weight, and after weighing my 04. 5 with it, it was legal but left only enough fresh water for toilet use.