An 18 gallon block of sludge? If you have a clothes washer there have been a few instances of postings regarding problems with tank possibly related to fabric softener and cloths lint along with other item washed down a drain. Doesn't happen very often.
If you can get to the tank measure it (length, width, height) to determine the volume. Fleetwood has on paper what should be there but verify it yourself. If the numbers match might be best to have a plumber run a fiber optic camera up the dump line, down the roof vent or a drain to see what's going on and determine the best course of action to take rather than stabbing in the dark.