Still have an older (~1996) Progressive Dynamics 12 Volt converter Model PD9055.
Never used in an RV, just as a power supply, and not used much.
13.6 volt fixed output. As far as I know, not up-gradable to multi stage setup. It is what it is.
$25 plus shipping. I haven't priced any shipping, but will find lowest cost option. (Probably won't do USPS - they seem really messed up right now.) No rush. Pickup possible.