The water tanks as originally installed by Hunter have no cleanouts. We couldn't see what was in there and we didn't like that. We wanted to se the inside, we wanted to be able to scrub them.
My method was to locate the baffle in each tank by probing inside through the gauge hole, then sawing a round hole in the center of each section. Where the section of the tank is covered with solid material (no lift out cover) I sawed a hole for an 8" round cover that can be pried out. Then at each locaton for the cleanout, I marked a round hole for a 6" plastic screw-in type access plate. Once that was sawed out, I could use some lifecaulk to make the installation watertight.
The scary part was that once I had the holes cut, I found that the tanks had GALLONS of crud in there. It looked a bit like a cave would with all the mineral deposits on the inside.
The other problem was that under these deposits the aluminum was very pitted, in some places nearly through the metal.
After cleaning out all the crud, I installed the covers. They have an o-ring like gasket around them, and that will seal much better if you put some food grade silicon grease on the rubber. it will also be much easier to close and open.
It may sound like we're done, but not quite. On the starboard tank, the vent is on the side. This means that once water gets up to the vent, there is a big cavity in the top of the tank that has no water in it. You end up short about 5 gallons in the tank. Which, by the way, is only 45 gallons capacity, not the 50 that Hunter advertised. You are really down to about 40 gallons in that tank. (Look for the manufacturer's label on the tank under the carpet and glue - it tells the real facts) What to do?
It is not really such a big problem, you just need to install a vent in the top of the tank. I went to the forward end of the tank, near the head, and drilled a hole in the top. Reaching through my newly installed cleanout, I stuck a small through hull fitting from the inside out the top of the hole. This fitting, which takes a 1/2" hose, has a hose installed and run to another fitting installed in the original vent hose up higher than the top of the tank. This simple modification will add about 5 gallons of capacity to your tank.
Coming soon - the port tank no longer has water - it will be a fuel tank in short order.