You'll get a lot of answers, with a lot of options, and they're all probably right. Google and read several websites on black-tank treatments/care/maintenance/tips. Research will help you decide what's right for you. But I'll give you my 2 cents as well.

We typically put a couple of toilet-bowl-fulls down the tank, so that's probably 3 gallons-ish, and I use a mixture of laundry detergent and water softener, not those green/orange/blue tabs. Just my preference, I'm no more right on this answer than the people who always use the tabs.

Originally posted by Hunter74       And I also figure when I go to drain it  (I have to drain it every time as that is the rules at my storage facility)  So even though I have the black tank flush valve I am assuming I should still fill it 2/3 of the way then drain. So can I fill the tank with the flush valve or should I pour water in the toilet with a hose or if I have any water left in the fresh tank use that?

You can flush it anyway that suits you, either using the outside valve, flushing-and-pumping through your clean water tank, stick a hose down the toilet, or all three. The one best advice I'll give you is what was given to me: use lots and lots of water. If you're the only one at the dump station, take your time and flush well. I'll sometimes fill my black tank full of water 3 or 4 times (if no one is waiting) and drain. 

Using this method I've never had a problem with my sensor and very little odor. I've got my own drain at home and I'll put a hose on the black-tank-flush and also use the Valterra Sewer Solution and let both run the whole time were cleaning out the r-pod, which can be 30 minutes of straight flushing. But that's just me, I like to rinse. Remember this phrase: The Solution to Pollution is Dilution. 

Originally posted by Hunter74     And after I drain the tank and rinse it out can I store it empty, or should I add some fresh water and another table thing when the trailer is sitting doing nothing.

You can store it empty, but realistically you'll never be able to get it empty no matter how hard you try. So it's usually best to put a bit of water in there with your additive of personal choice.