If yours is the original furnace, it is most likely an NT series furnace by Suburban. The dimensions may have changed slightly, but the Suburban is still the best candidate for a replacement. I had to slightly modify the ductwork and mounting area when I installed a new furnace in a 1965
Safari, but it was a rather easy job. For your trailer, check out the Suburban NT30SP. This is rated at 30,000 BTUs. Unit size is 12 1/2" x 12". Best of luck to you.