Welcome to irv2.
Here is a link
that will cover most of what you have asked.
You should check for good ground connections
to the chassis frame, clean and tight.
Your batteries should be checked also if not all ready done. Water level.
You should try another pedestal and see if there is a difference with long cord.
You could have a problem with the converter.
All things mentioned good clean tight connections.
Let us know how your making out.