Accumulator tanks are now required at the hot water outlet pipe of water heaters on residential construction and when replacing WHs in existing homes. This is to counter the expansion of heated water in the WH and hot water lines and prevent unnecessary cycling of the WH relief valves. So as mentioned introducing air into an RV WH or adding an accumulator tank in the hot water line should solve the problem unless there is a problem with the WH itself over heating the water and or the relief valve has failed to operate.
Although someone mentioned adding air by using a drained water hose on the city water connection, if you blow out your water lines to winterize, a shot of compressed air into the city water connection with a hot water faucet open will add air to the WH to reestablish the air pocket.