I have well water also. Notice that I didn't say AS WELL! I use the Pure Taste additive whenever I put water in my tank. Usually I have about 15-20 gallons on hand unless we're dry camping, then it's close to full. I never leave water sit in my tank longer than one month unless temperatures are estremely hot as this past summer. I changed it every two weeks just to be sure it didn't get funky.
As for cleaning up that tank I'd would use 1/2 cup of household bleach to every 15 gallons of water. Dump the old water, fill the tank about 1/2 way with the water/bleach mixture. Turn on your water pump and open one faucet at a time until you smell the bleach, don't forget the toilet. Then take the trailer for a ride and slosh the mixture around the tank. Once back let it sit for over night. Next day open the drain port and hook up the hose to the city side to move the solution out of the water lines. Once the tank is empty, fill it again, take it for another ride then dump it as soon as you get back. Rinse it out and it should be bacteria free and ready to be used. I recommend the Taste Pure stuff. WalMart sells it.
2007 Georgetown 370TS
aka - RAYNMKR