I had the same problem last winter. The smell would go away while driving but after awhile while plugged in to shore power it was back. Spent a lot of time on cleaning out both holding tanks, at least they were spotless. Then I found the problem, it was one of my batteries, the way I found it was, after plugged it to shore power for a while I could smell it again, so I went outside and went to the compartment where the batteries are and as soon as I opened it the smell was stronger. I placed my hand on the sides/ends of each battery until I found one that was way warmer then the others. It had a dead cell. Replaced both chassis batteries and smell gone. And yes it did set the propane detector off.
Here's a tip for pet owners, a dog passing gas will set the propane detector off.
Joe & Angie
Shih Tzu's Cookie & Rocky
2001 Tradewinds 7390 2011 CRV EX-L Navi w/ RoadMaster FuseMaster