The pressure after the regulator is only about 1 PSI, so it will not take a very strong hose to hold back the pressure. But it has to be rated for Propane.
Would there be another way to extend the trailer piping, by perhaps installing some 1/2" galvanized pipe (if the hose connects to pipe at the trailer side?) or extending the trailer pipe towards the new tank location?
You should be able to buy some propane flex line at the local hardware store, look for parts at the BBQ section. They sell regulators, and some propane fittings.
Otherwise check with your local propane supplier, and see if they sell hose and fittings.
Good luck,
Fred.