Thanks for the thread about furnace replacement.
I have not replaced mine yet, it seems to work fine, but I plan on replacing it before next fall for the following reasons:
1. Heat exchanger is SS versus cast iron, so it won't rust (thanks for the info idroba).
2. New furnace is more efficient.
3. No concerns about a hole in the cast iron heat exchanger (safety).
4. Spark ignition.
5. fan uses much less power to operate. I believe the new fans use only about 1/2 the power of the old fans, thus much less drain on the batteries when boondocking.
6. New furnace is probably lighter (like everything else).