The gurgling sound indicates to me that the pipe from the toilet to the tank has a buildup of solids in it preventing further fluids/solids from moving down into the tank. When this happened to us I thought the pipe went straight into the the tank as in pull trailers and that the vent tube was blocked. HOWEVER 5th wheel units have 2 45* fittings (one at the toilet and one at the tank) so there is a 2-3' section of 4" pcv pipe at a 45* angle that catches the solids and restricts their movement down to the tank.
Don't tell anyone in Yellowstone NP that I needed to cut a 5' willow switch to push the solids down into the tank when they built up in our unit. Since then we use alot of water to flush. The same condition arose in a cousins motorhome last year and back-flushing did not help until we went for another willow switch. ( which cured the problem.) The flexible wand flusher thing (Walmart) is handy to have but doesn't seem to move the solids down as well the flexible willow.