It does sound like a battery issue. If your coach will allow the slides to operate with the engine running, try doing that. On the diesel coaches, the slides run off the chassis batteries.
You may be having an issue with the battery charger bridging over to the chassis batteries, when on shore or with the generator running and the chassis batteries are trying to carry the load by themselves. If you can operate the slides with the engine running it will tell you one way or the other if it's a battery issue.
If you can't do that, try having someone operate the slides while you hold down the battery boost button connecting both sets of batteries.
Don & Mary
2019 Newmar Dutch Star 4018 (Freightliner)
2019 Ford Raptor