I would fill the tank with at least 5 gallons of water, wait for a day to let it soak in and then take a drive on a bumpy and curvy road several times. Some folks dump in a couple of bags of ice cubes to help break up stuff as it sloshes around.
Dump to see if that helps... a clear hose attachment helps to see what comes out. You can get one online or at an RV supply store.
An alternative (and will take longer) would be to fill the tank nearly full then dump in some Thetford Waste digester chemical. I prefer the powdered kind. Be sure to use the amount suggested based upon the size of your holding tank and maybe one extra envelope for good measure. Take a drive to slosh it around and then let it sit for a few days. Take another drive and then dump.
Hit the bu