overload shocks or helper springs??

If it were me, I'd take the truck and TT to a scale and weigh it all up. Find out what your actual payload capacity and tongue weight is. I think it's better to know your weights and capacities before guessing at adding springs or anything else. What if it turned out you are over on payload capacity? (It's possible you are over to some degree, esp. if the 6500 lbs is dry wt.) Also, knowing your actual tongue wt. will allow you to confirm if the spring bars are correctly sized. Too small may mean you won't be able to transfer weight between the 3 axle sets properly. Could that be one reason the rear of the truck is a bit low?

I added Bilstein HD on our current and last truck. They are impressive and a major improvement on handling.