Single pane windows or dual? Now the obvious remote tech questions, any windows or vents open? Can you feel any cool drafts anywhere inside your rig? Are your furnace air intakes blocked or restricted air flow? Floor register ducts may be restricted preventing more vigorous airflow. Any one of those things will reduce the furnace heating efficiency thru blocked heat or heat escaping almost as fast as you are making it. RVs are notoriously "leaky" when it comes to either heat or A/C. Just few thoughts.
Sounds like your furnace is not the issue unless it is cycling off prematurely. it's not designed to run continuously, it takes "breaks" to avoid overheating its own components. Too often and it cannot even get up to proper operating temperature. Too long and it can burn out.
2017 RAM 3500 Laramie CCLB Dually CTD Aisin, OEM auto level rear air-ride, B&W hitch
2020 3813MS Legacy Cobalt FBP, MORryde 8K IS & pinbox, PI EMS,DIY mods by in-house "craftsman", RV security system Mk1 Beagle Alerter