check your hydraulic lines
For those of you who have "older" trailers like our 2007 3400RL: recently took our rig in to our RV techs that we have used since we bought the trailer in summer of 2006 (we trust them implicitly) and I told them to check the gate valves on the three waste tanks (two gray and one black on my rig) as they were getting harder and harder to close. With the age of the rig they recommended replacing them, which I was expecting. When they dropped the coroplast to do the work they found that the hydraulic lines for the slides had deteriorated to the point that they were leaking, just not enough for us to see below the trailer, and I assume the coroplast absorbed a lot of it. I am really glad they found this before we were at some remote campground and the slides would not go in or out due to leaks. We love our trailer and have maintained it and upgraded it impeccably, but just like a tow vehicle we need to expect to replace stuff as they age!