How long did you let the water sit in the tank before draining it?
If you have a years-old pile of dried out waste, it may take quite a while for it to rehydrate and be flushable.
I would fill the tank 3/4 full again with water and a couple cups of Calgon water softener and a bottle of Dawn dish liquid. Let it sit for two weeks and then take a drive to slosh everything around.
Flush when you get home. This should take care of the dreaded Brown Pyramid and clean your sensors as well.
Incidentally, the quickest way to BUILD the pyramid is to leave the black water valve hooked up and open at a campground. Waste simply won't flush out with the relative trickle of water that RV toilets use.
Always keep the black tank valve closed and don't dump until 2/3 or more full. If you have to dump before that 2/3 is reached, add fresh water; you need that WHOOSH to get everything out of the tank.
Add a few gallons of fresh water to the empty tank to prevent waste from sticking to the bottom.