Valves are made to withstand high pressure. It's possible your propane tank is nearing empty, thus not as much pressure. With the pressure relieved on the valves, it's possible it could start leaking.
Even a check valve made for water, if the water pressure is relieved, the valve relaxes. The potential for water to flow backwards is there as check valves rely on sudden and continuous back pressure to work.
It's possible you propane tank is just getting low. And since the camper has been sitting in one spot for a while, the smell just accumulated.
Try switching tanks, wait a couple hours and see if the smell is still there.
At least you'd know this way and can eliminate one potential cause. If it still smells after switching or filling the tank, then move to the next step (whatever that may be).