Good to hear that it's holding water, I had a feeling that was what you would find. As far as keeping the tank clean, my humble opinion is to not worry about it too much.
It's a tank full of poop water, it will never be clean enough to eat out of so just let it be. I sometimes run the tank sprayer in mine to help clean the tank out. But in the end you have no way of knowing that it's really clean and can only assume that it is. Just keep some water in it and empty it when full. Yes I've has some smell when dumping my tank, but really that lasts for all of 5 minutes then the wind blows it away.