Furnace BTUs 18 vs 30

You may want to recheck the numbers (or look at the furnace as manufactures do make typo's) as I would think that, any rig with a Four Seasons "sticker" would have the larger furnace. Regardless though, it is doubtful that that you will have 18 KBTU of heat loss unless you are camping in really cold weather (less than zero). So the difference will be how quickly the rig heats up. The air coming out of the lower input furnace will feel colder and the trailer won't heat up as quickly so the furnace will run longer to achieve and maintain the same temp. But, in the right conditions, this could be an advantage as the furnace won't cycle as much (it will stay on longer) so the temp will be more uniform (once the trailer heats up). If it was me, I would look for a two stage furnace that had a two settings (18K and 30K) as the fan would likely be quieter when running at the lower setting.

I still bet it's a typo though.

Happy shopping,
Steve