Sounds to me like the charging circuit is not working and the battery died, and then you have restored 12v power by hooking up a battery charger to the battery.
WIthout any other information that implys that you do have a short somewhere that is drawing the system down AND your converter is burnt out or malfunctioning in some way.
If you can find the fuse to the slide out motor, I would pull that one first and then check the battery voltage both with the external charger on and then off. Report back on that (flip off the converter breaker for this test). Then flip on the converter breaker and check the voltage then and see what you get. What make and year of RV do you have, I don't see it in your signature.
There is alot that can be happening with your problem, but the above should isolate whether the slide is pulling current or not and if your converter is working at all. There are some additional fuses in the converter that also may be a problem. You need to see if the converter is working and to see what maybe causing the current drain. The slide is certainly suspect, but there may be something else at the root of the issue. If you are in Denver, I may be able to help more directly.
2007 Chevy 2500HD Classic LBZ Duramax/Allison (Stock)
2005 Bigfoot 1500C8.2 AWESOME