In case anyone is still interested in this subject:
I may be on the road to a solution!
A very clever fellow over at the Open Roads forum has posted his truly brilliant design for a simple heat exchanger he's rigged up for his camper. His purpose of course is simply to provide more heat for his unit, and not the production/recovery of water.
MY attention was certainly arrested when he "warned" folks that...wait for it....the unit produces water as a "byproduct"!
His design does use some additional power as two small computer fans are involved, so as designed it doesn't fit my Rigorous No-New-Power parameter...but I may be able to overcome this "flaw" by the use of a second Ecofan.
Anyway, his invention is truly brilliant just as it is, and the discussion is a recommended read for interested parties:
RV.Net Open Roads Forum: Sleep with warm fresh air in the TC using a heat exchanger