It does sound like you have restricted the return air and by doing so the furnace is now operating on the high limit switch because the temperatures of the heat exchanger are too high. THe furnace must be able to take air into the box from the vents on its side and move it across the heat exchanger and blow out the duct work effficiently to prevent it from getting too hot and shuting down prematurely on the high limit. If you will cut away some of the foam insulation from the furnace until it operates normally, you should be fine. Good luck and let us know how you do.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7