I generally totally drain my tanks in the winter, so water in the valves and drain pipes won't freeze.

Once it is spring, though, once my black tank is dumped and rinsed, I put chemical and 5 gal of water in it- and leave it that way until I use it.

Fresh water wise, I'm told 1/3 of a cup of bleach- the generic stuff that doesn't have brighteners or purfumes- per 50 gallons is about right. Too strong, and it's tough get the bleach to rinse out... I let the bleachy water sit in there for several hours, then rinse a couple times.

Regarding the sulfur smell in the fresh water, how was the water that you filled with?

 

I bought it used. the tank was empty, and when I filled it up it smelled like sulfer. That was the 3rd used RV with sulfer smelling water. It has a 100 gallon fresh water tank. I cleaned it with a lot of bleach (5 gallons), and it took 100's of gallons of water to get rid of the bleach smell. But now it's good. Ijust don't want to go through the same ol mess again.