A couple more thoughts - this happened(s) to us as well:
1) Where the washer drains into the pipe stand - the washer drain hose is placed into the pipe stand. FR forgot to put a rubber cap on top of the pipe to seal around the washer hose so I bought a rubber cap (for a 1.5 or 1.25 inch pipe - can't remember) and punched/cut a hole in the top to place the washer drain hose through. Initial problem solved.
2) Sometimes when draining the gray tank, the vacuum created sucks out most of the water in the p-traps and we have the smell immediately when driving. We just run some water in the sinks and tub to reseal the traps. That fixes that problem for us.