Opening the rear window is not a problem. You open both hooks and the open the window by just using the left lever. The window portion within the bathroom will be frosted for privacy.
Regarding the water tanks, the original had only 2. Fresh water was underneath the front couch And black was under the toilet and both were above the floor.
Now I have installed 3. Black is under the toilet in the far rear, grey is just behind the a le and fresh ist just in front of the axle. I have not measured my tongue weight yet but you are having a point in being careful here as I'm towing with a VW Eurovan who has a limit of 400 lbs. Not much room to play with.