The only fix I'm aware of is to use your black tank flush, if you have one, or to repeatedly FILL and drain the tank with clear water until you get rid of everything. This is probably best done on a site with full hook-ups as ideally you want to let the water sit for an hour or two between the fill and the drain so it has a chance to start breaking down the next layer. It may take a couple of days to get back to the bottom of the tank, depending on how bad it is.
If you do have a black tank flush and haven't used it before make sure you read the instructions first. You do NOT want to accidentally pressurize the black tank.
When I drain my black tank I always start the next cycle with five gallons of water and the recommended amount of TSP for my tank size in the black tank, even when I'm boondocking. I just set aside five gallons of grey water when I empty that tank. That way the bottom of the tank under the toilet is never dry, even when it is at the high end of the tank like it is in my current trailer (i.e. the toilet on the opposite side of the trailer from the dump valves).