Sounds like you may have "fixed" your problem.
I "sanitize" my tanks after most camping trips, especially when there is going to be a long time between trips.
I pour in about a cup of chlorine bleach into black and grey tanks, then fill the tanks with water. Let them sit for about an hour. Drain tanks.
This works for me, and I am able to do it at my house.
Even when dumping and you don't have the time for full tanks to sit for awhile, just try adding some bleach after you dump and are rinsing the tanks.
And, of course and always, use plenty of water with the black tank. I fill up the black tank just before dumping, even it isn't already full.