I had that problem too. I've replaced the ABS pipes under the kitchen sink and the shower. Unfortunately I saw the shower leaked on the last trip but forgot to fix it and the storage area under the bathroom got hosed when my son took the first shower yesterday. I think the problem may have been that they used compression fittings and over tightened them. After a few trips of shaking the whole system, the fittings cracked and broke off. I replaced them with pipes that screw directly on the drains and used some Teflon tape as an extra seal.
I just got back from the campground today, but tomorrow I plan on checking all the plumbing out I will fix the problems under the sinks but it is too hard for me to reach the pump that will go back the dealership for repair after all it is still under warranty. other than these problems the unit is very nice and we do not regret buying it.