SF42 Furnace Running - will Heat for a few minutes then cold

FINAL UPDATE- at least until the next issue LOL- New Thermocoupler resolved the issue- we have HEAT!  Now we will make our way SOUTH where we hope to not need it- or very little of it anyway.  This was my first time using the forum in the Good Sam Community and I must say all your responses were great,  gave us lots of things to trouble shoot, and I appreciate each and everyone of your responses!  I wish you all a Merry Christmas and Safe Travels if you are out on the road!