Not following the story very well :( Anyway, just to clarify, you have one of these, which has the converter and power centre under the fridge (in photo there?) and it has a Chev 454 engine? Where is the house battery bank and what is it made up of?
No idea, but sounds like your driver's side batt is an extra "aux batt" and the starting batt is is on the passenger side. What is the middle batt mentioned?
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EDIT--are the driver's side and "middle" batts the two house batts? what are they (brand and model) and what is the brand and model of the passenger side "starting" batt? What is the brand and model of the new driver's side batt and how old is the middle batt?
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http://www.mhsrv.com/1998-tiffin-allegro-bay-w-slide-m34-used-rv-for-sale-used-class-a-tx-i485036Not clear you did an actual "bench test" of the converter--ie not connected to anything, and when plugged in, the output was 13.6 volts.
When you do measure "converter output" where exactly do you put the voltmeter probes? At the converter itself, or to the battery lugs on the DC fuse panel, or to where?