The gauge does not tell you the level of the gas in the tank, you have to weigh the tank to determine that. Weigh the tank and then subtract the empty tank (tare) weight. The gauge is there as a safety measure.
To test the system, operate the stove
, then close all the burner valves, but leave the solenoid switch on. Read the pressure gauge, then turn off the manual valve on the tank. After 3 minutes, read the pressure again. If it is unchanged, wait 15 minutes and read it again. Any drop in pressure indicates a leak that must be located (with soapy water) and stopped.
I would be very careful with your setup. This is one area where the ABYC standards are more stringent than the EC standards. I recently read about (in Cruising World, Sailing world??) a UK boat that blew up because of a propane leak. The boat was at dock and when the cruisers were making breakfast.....BOOM. It seems that the EC standard allows the Solenoid to be anywhere from the tank to the device using LPG, and in this boat a leak apparently occurred somewhere between where the tank was and the solenoid. The ABYC standard mandates that the solenoid and the regulator are right next to the tank. A much safer setup.
Take a look here for a little more detail. http://www.boatus.com/boattech/casey/18.htm
Either way, with your set up I would definitely have a propane detector on board.