The Red Regulator. In my 5th wheel (specifically) there are two 30 pound tanks; one on each side. There is a 8 foot long crossover hose that transits the front storage compartment through a plastic tube.
On the door side of the camper, the propane compartment has the automatic switchover LOW pressure (Silver) regulator. That regulator has a one way check valve on each
propane tank inlet that is VERY easy to damage and is also susceptible to sticking due to condensation and oils from the propane. On one side of the silver switch over regulator is the short 18 inch pigtail (with Acme Nut) that connects to the one way check valve. The other end goes to the 8 foot hose to the other side.
On the other side of the 8 foot hose, there is a RED Higher pressure regulator and connected to the inlet of that red regulator is another 18 inch pigtail with Acme Nut going to the tank. That red regulator drops tank pressure to 18 PSI (the only replacement I could find drops it to 20 PSI.
While not required for operation of the system (It worked fine with a union in the line till the one I ordered came in), apparently the red regulator is required by DoT regulations for travel on the road when the tanks are separated by that long hose.
I had to special order mine from Lazy Dayz RV when I was in Tampa because NO ONE seems to carry the threaded high pressure regulators on the shelf. The home store (Lowes or Home Depot) ones have crimped on fittings and can not be made to work. The correct part was cheap (13 bucks).
Hope this helps.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW