Can I keep my propane tank valve open?
There are two possibilities:

Either you make the tank valve available with an access hole or similar,

Or you install an external valve, but this would defeat the logic of having the isolation valve as close as possible to the tank with no intermediate piping.

Making alterations to the gas system may invalidate any insurance coverage unless you have it inspected by a relevant authority - depends on the location ...  Lots of regulations for gas systems as the possibility of a big bang is high :)