OK - thanks - didn't think about the higher pressure on appliances ... will use soapy water test.
I have a suspicion it's the regulator and propane is bleeding into the camper thru the wall penetrations. It's a Lance truck camper, so has the box inset into the camper sidewall, not outside like a TT ...
When we shut off the propane at the tank, the smell disappears in a few minutes with the exhaust fan on briefly. When I open the valves, I can smell it just slightly - and then it smells inside within a few minutes...