We have a 2007 Sea Breeze, it seems they redesigned the plumbing since yours was built. My black-water tank is directly below the toilet and I don't have any back pressure issues. Have you thought about running a plumbing snake down the hole to se if there is a blockage? How about the tank vent stack to make sure it's not blocked. Lastly, you might think about asking the girls to put more water in the bowl before using it, that way more water will carry the waste through the system. Just my thoughts, good luck.
