[JRTJH]

I don't understand enough about LP gas plumbing to explain it fully, but the location of the regulator is probably the most important part of installing a "Tee'd pressure regulator" for a fish fryer.

LP Gas "evaporates" at the top of the tank and that "gas" (not liquid) propane is what is supposed to flow through the lines. On RV's with propane tanks that are installed on opposite sides of the trailer, a "step-down regulator" is used to prevent the high pressure from flowing through the lines. Under some conditions the gas in that "crossover line" can "re-liquefy" in the line and cause the regulator to fail.

If you're going to install a "10PSI regulator" on a line "Tee'd" at that tank, I'd think the regulator should be "at the tank" not "at the fish fryer", so installing a high pressure line on a T at the tank and connecting that to the regulator at the fish fryer will probably cause an unsafe/unreliable condition. There should never be any liquefied propane in the hose. If the hose pressure from the Tee is the same as the pressure in the tank, there's a chance the propane will re-liquefy.

I'd talk to an expert in propane system installations (not necessarily the counter guy at the propane company) and discuss the best way to proceed. Alternatively, as Javi suggested, just disconnect one of the propane tanks on the trailer and install the fish fryer on that tank. When finished, store the fish fryer and reconnect the trailer gas line. With a long enough hose, there's not even a need to remove the propane tank from the trailer storage rack.