I dealt with this problem years ago and learned that the funky smell and gunk was actually organic material decaying in the tank and falling out of suspension. The marina water supply was not chlorinated then and lots of "natural" algae and critters came along with our water. The chlorine I added to the tank killed them off inside the tank and they decayed and collected at the bottom of the tank. Here are my suggestions:
1. Kill the buggers and filter them before they get into the tank, or
2. Don't add any chlorine but flush thoroughly before and after you use the boat and empty the tank when away, or
3. Move to a marina (like mine) with chlorinated and filtered water supply.

p.s. When I first get to the boat I put the hose in the fill and open all faucets and the shower hose. I let it run several minutes until I smell a faint chlorine scent at each tap. When I leave the boat I drain the tank to prevent the water from getting stagnant while I'm away.

Chlorine in water from your tap dissipates in an hour when exposed to open air pressures. Chlorine may not kill a lot of algae, filtration is better for algae removal. DRINK BOTTLED WATER OR AS THE OLD BRITISH NAVY DID WAS TO SANITIZE IT WITH RUM. Really bring your own drinking water, or boil it for ten minutes to keep from getting Montezuma's Revenge. Gallon jugs are for cooking, small bottles for drinking. Hot water, 160 degrees F., is for washing dishes. Use disposable plates and glasses.

If you ever saw a lake or river water sample petri dish full of all the critters growing in it while it incubates you would drink only beer. Well you can not drink alcohol while underway without endangering your job and lifestyle as Roscoe P. Coltranes are all over there looking to enrich the local coofers so they get an attaboy.