Wayne
You should really consider adding a Marine watertight deck plate to the top of the water tank. It accomplishes two things .
1. You can open it and use a wet dry vac to totally drain it dry. This is very nice if storing for a while as the water will eventually grow mold.
2. It allows you to easily get your hand ( and arm) in to clean and sterilize the tank without having to run loads of water through it to flush it out. I have done this in my last few RVs and boats and it is my first mod. See Link below.
http://www.westmarine.com/webapp/wcs...classNum=12946