I can tell you what Click and Clack the Tappit Brothers told one caller to Car Talk who had that with their car... Replace alternator,, and that should work.
On an engine that old, Serpertine belts do not normally slip, but the old "V" belts as they age stretch a bit, and if they get loose...
I had 2 cars and helped 3 other people with the symptoms you had, Re-tensioning the belts was all that was needed.
(That is the last step in replacing the alternator).
Also.. A thought... Some RV's like mine all batteries charge no matter the source, (Bidirectional Isolator Relay Device)
Some do not, some the engine is supposed to charge both house and chassis, but the converter (house) only charges the house.
A Trick-L-Start adds the reverse missing function (House charging of chassis).
