[sourdough]

Since you've ran the trouble past the check valve into the line feeding the system you are now looking at having to do some "intrusive" stuff to the trailer. If it is coroplast (hopefully) it is highly possible that you would have to cut the coroplast to get wherever you need to be. The pex fitting could be compromised or the flush mechanism itself. Trying to dig into that could mandate that you do something that would not only compromise the trailer but also your warranty. Is taking it to the dealer out of the question? If so, we need to proceed on with the best way to access those components just keep the above in mind.