ikes1234 is the MH new to you and is this the first time you have fired up the heater. Just that to have two independent thermostats fail dangerous at the same time is unusual and I wonder if a previous owner has done some rewiring.
Another alternative is that the safety valve has gone faulty and is blowing off well below the temperature at which the thermostat would cut out. Is it going off with noise and blast of water or just quietly opening and letting water flow.
If you have an electric option, have you tried it on electric to see if the valve still blows off.