Again Mad dog is right. It could be a bad connection toon somewhere causing voltage drop and you need to start at the generator and work your way towards charger if indeed the voltage is low at the charger. You may find several bad spots with each causing a small drop but they will add up. This won’t take but a couple minutes but I would check at the charger when it isn’t working to verify voltage first.

You will see very little or no volt drop at any length without a load. Formula for VD=I(amps)X R(resistance of conductors). It's linear.
BW

First off I would try to get charger to act up again and take a voltage reading at the charger, may need a splitter for that.
My point is to plug everything in like it was and start as close to generator as possible and take voltage readings then work your way to charger and see if anything shows up.
I had issue years ago using power at the track and my turbostart charger would only go up to 12 amps after a pass when it would normally got to 25. It turned out to be voltage was down to 100 volts from everybody running their ac, this is why I also use my Honda for charging battery in my car at track.

Thanks guys. I'll let you know what I find out