We were connected to a 250# propane tank while wintering and hosting at a forest service park. They just installed another valve in the big tank on the motorhome, ran copper piping from the pig to that new valve. Shut the normal propane valve off and all propane was then supplied thru the 2nd valve from the pig. DH did all this and I don't know the technicalities but the propane regulator that is already on the tank still regulated the gas pressure. If we ran out, we could just change which valve was turned on to change the supply. We are currently parked and didn't get a chance to fill the large on board tank, so are running with a 20# tank that we can just refill. That is now connected to that 2nd valve.
