Thank you. Much of what I write is aimed towards educating others rather than simply addressing someone's specific situation. (I know and recognize a lot of what you're doing ... much of my life experience has involved 'reading between the lines' to see and recognize what is not readily apparent.)
Most (all?) of what you said is very valid and correct. I will add some comments, more for others than you.
Regarding your experience with your '91 BMW -- there's two interpretation of POS -- 'previous owners' or 'pieces of ...' (Both apply to some people.)
Little known is the increase of electronics in vehicles over the past 20-25 years creates a small but steady parasitic drain on batteries. Implicit in the design of these vehicles is the assumption the vehicle will be used every 7-10 days, allowing the drain on the battery to be replaced regularly. (i.e., the battery will be recharged through regular use before the battery is completely drained.)
There are means to offset or prevent this situation but those would mean an increase in vehicle cost ... and 'round the flagpole we go again.
Stark evidence of this is every time a military unit returns from a lengthy deployment or exercise. (There's two large military installations in the Fairbanks area.) All Fairbanks stores selling batteries consistently experienced a huge stream of customers seeking new or warrantied replacement batteries. This was especially acute in the winter, with many batteries frozen and even split open. (My military contacts indicated many cases of explosions on base/post when troops tried to recharge frozen batteries.)
What you're experiencing and going through is 'been there, done that' for me. When I got my 1970 Explorer Class A (on a 1969 M300 chassis), I didn't know a **** thing about RV power converters. (General knowledge about power supplies and battery chargers but nothing specific to that application.) I 'murdered' several batteries before I figured out the power converter was overcharging them. I eventually developed a work around to deal with things.
Where I am today has involved a lot of research ... and that research is ongoing. Some of that research has involve lengthy, in-depth email discussions with engineers and technicians at companies like Schumacher Electric and Blue Sea Systems. That's what I'm bring to this thread.
More 'robust' electronics means more cost ... so "you get what you pay for" is at least partially true. Higher cost is an indicator -- to a limited degree -- of better quality. Progressive Dynamics power converters are at the upper end of the price range. This, along with Leeann's experience and some internet digging on my part, leads me to believe their products are among the best available.
I have firm reason to believe some higher cost power converters largely eliminate the problems people experience with old and/or less expensive versions. (This assumes the power converter is properly installed and used ... it's impossible to eliminate the possibility of an inexperienced person -- or fool -- misusing something.)
Gotten run ... there's some pressing IRL issues I need to deal with.
