Welcome to the forum.
Sounds like it might be a thermostat problem. If you carefully remove the thermostat and then turn on the propane, you'd know if the thermostat was causing the problem.
Sherm & Terry w/rescues Steve (std Poodle) & Eydie (min Schnauzer)
2015 Jay Flight 27RLS (Camped: 102 nights 2015, 90 nights 2016)
2006 Ford F350 6.0 PSD, Lariat, 4WD, CC, LB, SRW, auto., Camper pkg.