ONnboth my 2003 and 2012 139 models, the whole furnace assembly was to the right of the kitchen sink and under the stove. The machinery itself was hard to access -- remove the furnace grate and you can see the works.
If this does not answer the question, let me know!!
2012 Fun Finder X-139 "Boondock Style" (axle-flipped and extra insulation)
2013 Toyota Tacoma Off-Road (semi-beefy tires and components)