Brian, problem number one: "Dealer told us" Very, very few salespeople will balk at a chance to sell an RV even though a few white lies are needed.
Read the cargo capacity on the yellow door sticker of your 3500 and go from there. Your pin weight is probably 2800 from the factory, closer to 3400 loaded and ready to go and then add fuel, family and miscellaneous for your pickup. The yellow sticker read more than 4000 pounds? Oh, better add in your generator too....
Jim and DW 50 years Brenda
2018 40rssa and 2021 Jayco Eagle 40'
2019 F450, ruby red
Harley Road King & sidecar