We are leaving our trailer at a private campground and each time we want to use it, they tow it on and off whatever site we wish to use. If we are camping say, every second weekend, would you empty your black and grey tanks every time you camp?
I'd empty the black tank each time I was getting ready to leave the trailer for two weeks. The grey tank shouuld already be empty if you have been hooked up while using the trailer. I leave my grey tank open while connected to the sewer line and the black tank closed. I would never leave the black tank for weeks at a time with you know what in it. Treat it also before you leave the camp grounds each time.
I would empty the black tank and then empty the gray tank to flush out your sewer hose. We keep our gray tank valve closed just for the purpose of flushing the black tank. Keeps the sewer hose cleaner while storing it. Just my opinion.
Leaving anything in either waste tank will get you some really rank odors after two weeks in hot weather! Before you dump the black tank, if it is less than 1/2 full, add water to it to bring it to that level. I would also flush out the black tank before leaving to keep left-overs form drying out inside the tank.
I'd drain both blackwater and greywater tanks each time out - might require you to add water to your blackwater tank to bring it up to 3/4's before dumping. In addition, I would disconnect the sewer line from the camper/sewer and store during non-use so the line won't be harmed by sun, animals, falling branches, etc.
I do mine like DL, I flush the black first then the gray. I have an extra hose that I use to back flush the black tank and then drain it again. Sometimes it takes 2-3 time to clean it out to my satisfaction, so for it is working.
We camp in a seasonal site ( that has a full sewer) so our trailer never moves. I usually leave the gray water tank open and about once a month, I'll close it, fill it with water then dump it, just to give it a good flushing. I leave the black water tank closed and I add 8oz of the "blue stuff" in the tank. Whenever it's full, I dump it and give it a good rinsing (I make sure to close the gray water valve first) then add the blue stuff and a little water.
This seems to work well for me, I don't have any odor although, when I have to go #2, I usually take a stroll over to the shower house and use the facilities there, that helps considerably on the odor control too.