I want to add a hot water recirc line to my trailer

All you need to do is run a shunt from the hot water line under the sink to the cold water line. Put a backflow check valve to keep the water from flowing backward, and a small water pump. Turn on the pump and it pulls water from the hot water line, and recirculates it back into the cold water line. Brings hot water to the faucet without wasting any water at all.