If the batteries are bad, then the charger is not going to be able charge them and bring them up to voltage. Your symptoms are similar to a bad battery.
How old are your batteries, what is their voltage after they are sitting at rest for two hours (nothing on drawing power, and the charger/converter not trying to charge them)? I'm betting when you get the battery voltage and compare it to a battery voltage table, you'll find there voltage is substantially low.
How is it exactly that your friends determined the batteries are okay?
DaveB, Raleigh, NC
2015 Tiffin RED 33AA
2014 Honda CRV