If it is the dreaded pyramid, its under the loo, or in the pipe down. Many MHs & 5ers have a 45 deg. offset from the biffy to the tank. It will plug up real quick if too little water, with too much waste is flushed. #1 rule- (for#2) fillthe bowl with water, flush, twice even.
A few times a year we see this problem, so simplest for minor plugs is to push it down with a stick, or down around the bends with a stiff hose, my favorite ( still yicky) for that is a length of 3/4 BX armored conduit.
We have unplugged all but the dried out ones with our phelps honeywagon adapting the suction hose down to 1" spa hose, sucking up the plug. Good luck!