Frustrating furnace issue

Forgot to mention, rarely are the RV's ducting adequate to disperse the volume of heat put out by the furnace. So, that hi limit switch will cause the burner to come on and off occasionally when the furnace is on a heat cycle. It will relight once the combustion chamber cools down slightly. You stated the furnace never got you above 40 degrees, so that points to a Hi limit being bad. To check for LP pressure and volume. LISTEN to the Furnace and you can tell when the burner is ON. You will hear a slight roar of the burner. IF the burner kicks off(the roar stops) and the RV is not close to heated up temp, light all 3 or 4 burners on the Oven range top. Let them run for 5 minutes or so, THEN turn the furnace tstat switch OFF and then ON. The furnace SHOULD light within 45 seconds. When it does light, watch the Range burners. If the flames diminish or go completely out, you have a LP problem. IF the furnace lights and the range burners stay lit and full flame you have a furnace problem. Doug