You should leave some water in the tank after dumping. I've seen people recommend up to 5 gallons of water. On mine I just add a gallon or two and call it good.
But also don't forget the toilet bowl. You want to leave a little water in the bowl to keep the seal from drying out and sticking. I keep a few one pint drinking water bottles in the bathroom. If I find the seal dry after extended stay in storage I just open one of the bottles and pour some water back into the bowl.
The bottles also work well if I have to flush the toilet while picking up the trailer from storage. It's amazing how my children can leave the house not having to use the bathroom, and suddenly have to pee when they see the trailer 30 minutes later. :h
Oh, and before anyone says it, these bottles are refilled with plain old city water. I'm not going to use bottled water to flush my toilet. :R
Last thing, If you do find a dry bowl seal, put some water in the bowl and let it sit for a while before you flush it. This way some of the water will hopefully seep around the dry seal and lubricate it so it doesn't stick and tear from being dry.
