When my Ford dually carries my 6000 lb TC just fine, on 1 trip when I had 200-300lb of waste water in rear tank, it was prone to porpoising. Nothing really major, but now I avoid carrying waste water and pay attention to loading heavy items in front, or truck cabin, while only light stuff goes to the rear.
For my summer camping 5 gallons of propane is like 60 days, so I guess your furnace used a lot?
When you can buy special valve to tap into RV propane line, those bring maintenance and reliability issues. I have small BBQ that uses small propane cylinders right on it and small bottle last for several times, when buying dual pack cylinders at Walmart come to like $3 a piece.
My friend is having 2.5 gallons propane cylinder, where he bought a mast with T-s on it. Mast bolts to the cylinder. So top holds propane light, very efficient and then he hooks up BBQ to lower outlets.
This way he has light and propane without hose limitation.