I replaced the floor in my coach back in the 2000 or so an then replaced 1/2 of it again maybe in the 2010 time frame. I should have done the whole thing but now I am looking at doing the other 1/2 before the tanks drop out! I used 3/4 marine plywood as that's what the original configuration was. There were drains from the tanks going through the floor but there were no drains for water leaking into the compartment. I think you will find that this is a fairly normal configuration. I am going to have to pull the tanks and put in a new floor in the 1/2 I did not but dang, I really hope to put it off until I see the tanks hanging down from the underside!
