Though I have heard of tank to tank transfers they are "Discouraged"
A much better solution is called the "Extend-a-stay" This is actually a brand name of a product once made by a company (Marshall brass) my mother once did a consult for.. but alas I do not think they make it any more.. MOST RV stores if you ask for it they will take you to one of several competing nearly identical products. }
The full kit consists of a 4-way manifold that goes between the tank and the regulator. This manifold has 4 ports. The main inlet port is a POL fitting to match your RV's tank, the main outlet is either an ACME or POL to match the fitting on the regulator. The other ports are a "Quick Connect" (like the top of a disposable bottle) and an "inlet) small nut or bolt (Plug really) screwed into it .
Plus two hoses
one is a 10 foot Quick male/female for running a portable grill off the RV tank
The other is a short (5') hose Acme to that secondary inlet (the one with the plug)
Check valves insure propane only flows out the outlets and other valves only when a hose is connected to the aux-out (Same as the bottle it resembles, same kind of valve)
Had one on my Rig for 15 years used both of the Aux ports at one time or another.
