1st trailer... septic hookup questions

If it is just you put a a few gallons of water in the blank tank. Use the toilet like any other toilet til it is full. Then dump. You want plenty of liquid along with the solids. Now you could maybe go 2/3 full then dump. The grey I would leave open until a couple days before dumping the black. Then let the grey fill up and dump that right after the black tank. The grey water will help flush all the solids from the blank tank down the line. The grey water will fill much much faster than the black tank. Under no circumstance should you leave the blank tank valve open. You will end up with a pyramid of solids where the goods hit the tank bottom. They will continue to mound up as the liquid flows out if you leave the valve open. Not a good thing.

You don't want to let your black tank or any tank for that matter to freeze. If you have tank heaters make sure there is some water in the tanks before you turn them on. That is if they are the electric pad type.