How heavy is your water tank? Well mine weighs 741 lbs full.
First thing that I thought about when I saw that statistic on the door of the RV was… I hope they have a strong support system for that.
I was first alerted to the weight last year when we broke our leaf spring in New Mexico. Read that blog post here. The gentleman who sold me a new one mentioned how dumb it was for a manufacturer to put something so heavy at the tail end of an RV. Putting a tank that far back puts an amplified amount of stress on the mechanics of a trailer due to the cantilever effect. Well it concerned me enough to at least look at what a half full tank looks like underneath. 
As you can see the tank does sag quite a bit in the middle, and seems to only be held in place with a couple of metal straps welded at each end. 
Upon closer inspection, I could see that a small amount of the connecting weld material was pulling off the frame. (essentially the weld was breaking)
So off to Lowes to see what kind of 8ft wide brace I could buy and add to the underside to help keep support on the tank when it was full and ease the stress on the factory straps.
Above is a side shot of how far below the frame line the tank sags with only 1/2 full of water!
I really wanted to use 1/8″ steel angle iron but they didn’t carry anything in 8 ft lengths to reach across the width of the frame. So I settled for this stamped perforated steel angles, but figured I should double them up for some extra rigidity. We marked them and cut them so that A bolt could go through both and the ibeam of the frame. 
After bolting them together and jacking the cross bar up with a floor jack. We were able to mark and drill the frame hole. 
So after the assembly was complete, you can see below that even the steel bends, so I think I am going to look out for real 1/8″ or 1/4″ black steel to build a more secure one, but for now it definitely helps support so there is no fear of traveling with a full tank now. (Except the stress on the frame, springs, wheels, axles… LOL ) 
