Yeah, I don't know how putting a 6V golf cart battery in place of a 12V did anything. Also, a battery never has "no voltage", especially after only 28 days without a charge. This would indicate either a problem with where you measured the voltage or the meter.
No offense, but you really need to get off the internet and get on the phone. You need to find an RV tech that an figure out your issues. Even if we were able to remotely diagnose your issues, I don't think you have the technical acumen take the corrective actions necessary.