Something that I thought existed, but I can't find now.
I recall, but it could be a figment of my middle-age mind, a nylon fabric cover that goes over the propane regulator/valve to protect it from dirt and debris. I've come across hard plastic covers, but I like the simplicity of a fabric boot over the regulator.
I know most of you have propane tanks mounted in protected locations, but on my teardrop my propane tanked is mounted on a bracket on the side of the trailer, so I want to cover the regulator when towing the trailer.
This is the closest thing I've found, it's a cover for the regulator for a Powertank, but it's sized to go over the Powertank handle too, so it's rather large.
Another possibility is a small neoprene pouch designed for camera lenses and such.