Clorox won't clean anything out, it will just sanitize whatever is in there. If the algae or whatever is in there doesn't get completely saturated/penetrated by the sanitizer, it will not be completely dead. The accepted practice for sanitizing food contact surfaces is to first wash and rinse, then sanitize. The tank on the camper is really just a big water bottle, and anyone who has kept a water bottle for any length of time know that even if you are just putting drinking water in there, over time, it gets funky.
esimmers: If possible, I think a better option would be to remove the tank from the camper and clean it. It seems like even if you had an access hatch, it would be pretty hard to thoroughly clean the tank, and it would probably mean a lot of spilled water in the camper during cleaning. But I have installed hatches on kayaks before, and that process sounds like what you are talking about. A backing ring is inserted through the hole into the hull of the kayak, and that gives the hatches screws something to grab as well as add rigidity to the hatch area. For rectangular hatches, the backing ring can be inserted whole; the backing ring for circular hatches has a cut all the way through it so the ring can be compressed enough to fit through the hole. Rectangular hatches are a lot stronger in my experience because of this.
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Couple of comments on this..one, the objective as I understand it is to "sanitize" the water systems in our FWCs...simply put, to kill algae or bacteria within the system...Clorox correctly used will do that...I have many water bottles and CamelBak bags that are kept clean in this fashion...without a "scub down"...deceased algae and bacteria will be flushed out of the system during the process.
Next both of my sea kayaks have circular through the hull hatches...non of these hatches required cutting the inside ring to install; you simply access the under side of the deck through opening where the paddler sits...plus it is my experience with open ocean touring that the round hatches [screwed to o-rings] will not leak as some non-round hatches will...cutting a round or square hole in the FWC tank will potentially require mods to the hatches and sealing surfaces.
Anyway to each their own, but doing major surgery to my Hawk to access the inside of the water storage tank is not in our future. I see it as a non-problem looking for an ornate solution that could generate not only a lot of work but could introduce potential leaks inside the FWC.
But hey, I could be wrong!
Phil