The more information you can give, the easier it is to help you. I saw a Damon Astoria on a Freighliner chassis.
You said it wouldn't start or turn over. Do the driving lights, wipers, HVAC fan come on? Perhaps your chassis battery(ies) are disconnected. Inside near the entrance there is often a couple of switches for battery disconnect switches, perhaps labeled 'Store' or 'Use.' These switches need to be on.(Use) Chassis batteries are for all driving functions, the House batteries run all the living functions, furnace, A/C, water heater, refrigerator, water pump, etc.
Check for voltage at the batteries if the disconnect switches are on. Also, have you tried the 'Boost' switch on the dashboard to connect both battery groups together for extra oomph? This button has to be held down as you attempt to start engine. In addition, you have to have the transmission in neutral to start engine. Some RVs even have an interlock that prevents starting when jacks are down.