Yes - both on-board converter/charger units and the battery are both used to power up your 12V Distribution setup. It normally works well with either the Shore power 'ON' or just the battery.
I have had that happen here as well. The slide requires alot of 12VDC CURRENT to operate. The normal 55-amp DC capacity from the on-board converter/charger unit is sometimes just not enough capacity to operate the camper slide-outs freely. Other 12VDC high current items may have been 'ON' at the same time...
I have on occasion actually nosed my truck up to the front of the 5th wheel trailer I was using at the time and used jumper cables from the truck to give a better battery assist.
To me its just the nature of the game. You can purchase a larger capacity CONVERTER/CHARGER unit I guess or just always have the Battery connected when in use.
My trailer setup is using smart mode converter/charger so having my batteries always connected does not harm the batteries like the single mode older converter/chargers would do. Batteries would tend to start boiling out fluids if left connected for long periods of time when using a single mode 13.6VDC system charging setup.
I learned all of these short comings early on and try to have a PLAN B to over-come them as they raise their ugly heads haha... "These short comings are not going to ruin my long term trip" - Can't always run back to the dealer sometimes...
Just some of my thoughts here...
Roy Ken