Sowbug... your battery may read 12.8 , but when a load is put on it, it may drop a long way. Pull the battery out and take it to a Auto Parts store and have it Load Tested to make sure that there is not a substantial drop in the battery voltage. Plugged into shore power or if no shore power , plugged into a running tow vehicle will help with the voltage a little to move the slide with a higher voltage. If you do have a shorted out battery (could still say 12.5v with a bad cell or two) and need a full jump of steady 12v.... turn your tow vehicle around, keep running, and use jumper cables on your battery to get the full power that you need to take the slide in or out (have shore power unplugged to be safer when jumping from the tow vehicle).
2013 Forest River Salem Hemisphere SBT312QBUD
2001 CrewCab F-250 7.3 PowerStroke Diesel
SuperChip, BTS transmission, 6.0 Trans Cooler