[MarkEHansen]

The battery is the primary power source for running the slides, awning, tongue jack, etc. as these are high current items. Having the trailer on shore power will add to the available power, but the battery must be in good condition and (reasonably) fully charged.

Note that when the battery is going bad (due to age and/or misuse), it can read a high voltage when there is no load, but read very low when under load. You can place your volt meter on the battery terminals as you test the slide and see what the voltage drops to. I think it should not drop below (I think?) around 12.2V

There's a lot to learn about the care and use of RV batteries. Please ask questions if you have any.