Personally I wouldn't jump to the conclusion the batteries are the issue. If the gen set started (good sign) and the inverter is charging you should have enough juice to operate the slide. If in doubt operate you battery parallel switch and see if that helps.
When the slide stops is the hydraulic pump still running? If the pump stop the issue may be in the slide control like a bad switch. If the pump is running than look for a stuck solenoid valve.
If you have jacks, are they functioning OK...most use the same hydraulic pump? So if the jacks are working the batteries are not the issue.
Are your the air bags at ride height and jacks up? If not and the chassis is a bit torqued large slides can bind in the opening.