I have a 2006 Warrior.
When you plug into your house, your batteries are charging.
When you go to check them after you unplug from the house, do you have a bunch of lights on or anything?
I can unplug my trailer, walk around to the door, turn on 2 lights and the batteries show yellow, brand new batteries.
I think you are reading too much in to it.
Maybe your batteries are getting old?
4 years is pretty good on batteries.
Do you have 12V or 6V?
How many?
These all play into the equation.
I have used the stock converter for years, have 1300 hours on my genny, no issues.
Sure there are nicer ones out there, but it works.