The vapor-tight compression fitting that SD showed is definitely the way to go. By the way, they are VERY easy to install, so no worries in that respect.
I would still advocate for a remote solenoid shut-off. In fact, I'm not sure if ABYC makes any distinction as to where the tank is mounted -- I think the remote switch is required regardless for any propane installation. If so, you should follow the requirement as it can affect insurability of the vessel, and it's the best practice anyway.
As an aside, I don't believe closing the propane valve (whether at the tank or solenoid) while continuing to operate the appliance "empties" the line of propane. I think this simply stops the flow of propane and extinguishes the appliance. If new propane is not entering the hose from the supply end, a vacuum is created and the flow simply stops.
But there isn't enough volume of propane in the line to worry about anyway. The worry comes from not closing off the supply. Which is why the remote shut-off is so important.
Make sure your hose is continuous all the way from the tank/solenoid to the appliance. No splices or extensions.
As for mounting your tank to the stern rail, you might consider an arrangement that looks a bit cleaner. These folks mounted two small tanks in brackets to the transom: