Cargo Trailer Conversion - Propane Issue

Yes, you can buy separate regulators so that you can tap between them to get the approximate 16 psi to go to a remote quick coupler. One of my first RVs had a Marshall regulator that was actually two piece with a quarter inch nipple between the first and second stage. I added a tee, an isolation valve and a 25 foot hose with the connector like the disposable tanks, I forgot the CGA#, to run any kind of camp grill I want. I kept that regulator and move it from RV to RV. I've had to make some new viton internal parts for that regulator over the years but its still working.
As far as being legal… I've been gas appliance certified since 78 and Factory Mutual, Zürich, boiler inspectors etc. gave me their blessing to repair controls,safety valves and anything else I needed to repair on large industrial equipment.
Besides being totally cheap junk, the only special thing about these RV regulators is they have the changeover built in to the first stage regulator. It works by setting the lead tank delivery pressure to be slightly higher than the lag tank delivery pressure so that the lead tank will run out first and the decrease in pressure allows the lag tank to deliver the gas and the lead/lag indicator shows that the system is now running on the lag tank. Craig