Some random thoughts:
It is not a bad idea to have a qualified person do a manometer test every now and again (it will register pressure changes – leaks- in your system).
Also, be aware, that US rules for propane differ from the UK and EU, so US vessels getting surveyed in the UK or EU might have a hard time passing.
In my 10+ years in the UK and EU, I noticed (casual observation) that fewer local boats had a gas sniffer/alarm than I would have expected and also did not have a solenoid shut off at the tank. The recommendation was that the tank be manually turned off when not in use: certainly a very safe practice, but not realistic when the need is to go out in the rain or when heeled over 15+ degrees and bashing to wind.
To me, one or 2 sniffers in the bilge (depending on boat/bilge design) are cheap insurance, last a long time and pretty much ensure having adequate warning of a gas leak. (They should be tested regularly: I do mine by shooting gas from an unlit barbie lighter into the sensor.)
The solenoid in the propane locker is just added insurance and should be activated near the stove, but not in a location where one might have to reach over open flames on the stove to get to it.
All systems are best designed with a pressure gauge near the tank. A test of integrity is to charge the system with gas, note the pressure gauge reading and then turn off the tank. If the pressure decreases over the next 5-10 minutes, you have a leak.
My best, Dick Stevenson, s/v Alchemy