I don't think you will need to worry about the dreadful sag, but I think the weight dynamic of having the tanks back there and full could cause towing problems. Water weighs 8.34 LBS per gallon. Even a 25 gallon tank is over 200 LBS. I have to assume the original design was to help balance the trailer. Unless you have modified the running gear I would attemt to do the same with regard to balance. Proper towing is a combination of axle weight and tounge weight. I would keep this in mind as you redo the interior. If you want to put the tanks in the rear you can, but I would put enough weight forward of the axle to balance it out.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato