I bought a brand NEW 21VFB with a 30 gal FW tank and it did the same thing on the way home from Kentucky, where it was purchased. I live in Texas and had the tank filled by the dealer for the trip home. I did a quick 'look see" to check out the rig and noticed the tank almost coming loose from the cheap metal angles supporting the tank. After contacting the warranty dept, they sent 3 metal straps (plain galvanized metal about 26 ga) and no attachment hardware and no instructions as how to mount the darn things).
I didn't want to drill holes in the existing structure for fear of fatiguing the angles, nor did I want to take the tank completely out of the unit, so I purchased 6 stainless steel 1" strap hooks ($3.55 each), screws and nuts to attach the straps to the hooks, rubber insulation to prevent rubbing a hole in the tank, and supported the tank with the straps by hooking the new steps to the metal floor trusses. The weight of the tank is enough to keep the tank in place when empty. Be careful, though….check your 'top clearance' because the tank also expands upwards when filled to capacity and will push UP on the floor if there is not sufficient room for expansion.
I still have NOT been reimbursed for the parts I bought since last August, and i was 'assured' by the warranty dept. it would in 'no way' affect my warranty and I would be reimbursed for my expenditures.