Thanks for all the great information--I will be studying/reading up for sure. I'm glad to know that the unit I found on Amazon is overpriced and won't suit my needs ... :-)
I also realized that my original post is unclear--I haven't allowed my battery to LOSE 70-80% of it's charge, it was STILL at 70-80% (so only 20-30% discharged). My battery is pretty easily accessible, and I've been using my handy voltmeter to check (waiting until at least 18-24 hours after any charging). Since I'm not really using the camper over the winter (I'm driving it for day trips only) I thought that this was the time to learn how the electrical system behaves.
I'm not sure which model MagnaTek converter I have--the manual is for "900 Series Converter/Battery Charger" and covers several actual model numbers. In the specifications, all of the listed model numbers give DC Output (volts) 12.0 min @ full load, 14.1 max. @ no load.