If your tank is presently empty, fill it about 2/3 full of fresh water. Buy some RidX in a hardware, HomeDepot, Lowes - - using a bucket inside the rig, mix about 4 tablespoons of RidX with one gallon of water and pour it in the toilet. If you can, take the trailer for a drive of about 20 miles. Then let it sit for one month if possible. Take it for another spin at the end of that, then drain the tank. The RidX enzymes will liquify solids in the tank and allow them to flush.
When we are heading for home after a trip, the last morning before hitting home, we empty our tanks, flush the black, then put 5 gallons of fresh back in both tanks. We then give each a shot of RidX along with each drain so it can slosh in the tanks that last day of driving. It then goes into storage with that solution in them. It can do wonders for sloppy reading fluid level sensors also.
2003 Dodge HO 6 speed Laramie Dually, Jordan controller, RDS fuel tank, Air Ride 25K hitch
Carriage 36' fiver, Onan 6500, Prosine 2K inverter, 3 slides, MORyde IS and Kingpin