normal to see yellow flame inside the furnace exhaust vent?

What he said.

This could be caused by the air intake being clogged with wasp nests or spider webs, but it will cause rapid buildup of carbon/soot further clogging things up and it will pump out dangerous carbon monoxide like crazy, and can kill you and/or the poor folks in the tent next to your rig as you sleep. Do not use this furnace until it has been repaired by a competent technician.