An interesting though. Here's a do-it-yourself article about doing just that -- using the water heater to provide heat in the RV through an exchanger:
http://www.solarhaven.org/HPArticle.htm
And in fact, so-called hydronic heating systems are fairly common in larger RVs, like diesel pushers. Friend of ours have one in theirs, and they love it. Of course, it doesn't heat very quickly, but that's less of an issue in a pusher because you are already in the RV on the road, unlike a trailer, so it's already warm from engine heat.
Maybe a more efficient furnace is also an option. You wouldn't do it to save money though, you'd do it to save propane and battery power, both precious resources on extended winter campouts. They make residential furnaces now that are 90% efficient, and they exhaust through a PVC pipe. But they are larger and heavier than less efficient furnaces, so that might not be an acceptable compromise in an RV.
It looks like Atwood does make 2-stage furnaces, which presumably would be more efficient, but it doesn't appear to be a drop-in replacement for the one in the TM.
http://www.pplmotorhomes.com/parts/r...-excalibur.htm
Dave