TWO HEATERS??? one single heater will pull around 15A. With line losses and resistance you are doing good to pull one on a 30A service. You are pushing to burn up either the wiring or your coach. Get one of the cheap plug-in voltage meters and use it inside the coach. Measure at your house, on a low use circuit, then at the inside of the coach to find our your voltage drop. Then turn on the heater and see how much it drops. If you wiring is OK, you should get very little (<5) volt drop. Of you voltage is below 120V, you are asking for trouble.
