I can't tell you about the 346FLS specifically. What I can tell you is that upgrading the Solitude 310GK to the 8K axles with load range H tires changes the GVWR on the sticker from 15,000 lbs. to 18,000 lbs. I assume something similar would happen with the 346FLS.
On a related note, I hope you're planning on upgrading your tow vehicle, too. at 16,800 lbs. GVWR (without the axle upgrade), the 346FLS is too much trailer for a single rear wheel 3/4-ton truck.
Rob
Edit/PS: we have installed disc brakes on both of our fifth wheels... it reduced the stopping distance by almost half. You'll love them.