Hi I'm just seeing this post. I too have a 1983 G van RV welcome to the club. I agree with what was said before. When you start it cold is it going into fast idle? I replaced my alternator and starter. Old starter wasn't spinning fast enough, I went to a auto electric shop and had them fit a alternator with more amperage, run larger wires from the alternator to the isolator and to the chassis. Also new ground wires. I run two chassis batteries and two house batteries. I had this work done about 10 years ago. My normal voltage running is 13.7 to 13.9 volts. I mostly use my RV as a cabin now. She can sit for months and still start right up, I love that about her.
2015 VW Passat TDI SE, 6 speed manual
Past Diesels
1980 MB 240D, 4 speed manual
1983 GMC Vandura 3500 Nu-Wa 23' Class C Motorhome
6.2L Diesel, Turbo 400
1985 MB 300D Turbodiesel, auto
1992 Chevy C2500HD 6.5 Dieselturbo, auto
1994 Chevy C2500HD 6.5 Dieselturbo, auto
2010 VW Jetta Sportwagon TDI, 6 speed manual, sunroof