When you look under the slide when extended do you see a shaft with gears on it and a pair of bars with gear teeth on the bottom. If so this is a Lippert slide and you could have lube issues with the pieces that retract into the frame.
These support the slide when out and if the lube has solidified at all (usually when contaminated with normal dust and aging) you will need to give it some attention.
Probably easiest way is to pick up a can of spray "Slide Lube" or other Dry Lube spray. Extend slide and generously spray the support bars, gear teeth, and support bushings for the "pinion" (shaft with round gears). With batteries charged and converter on run the slide in and out a couple times. Be sure to let it rest about 10 minutes when in or out so you don't overheat motor.
Repeat this process a couple times and if you see any dirty "grease" on the mechanism, wipe it off.
Another cause of slow slide operation could be loose connections along the way from battery to slide motor. On my trailer the slide is powered from a wire that starts at the main disconnect switch under the Propane tanks. I found this connection loose by my standards. If I can move the ring type connector on the stud with finger pressure it's too loose.
If you can, hook a multimeter to a power source connected directly to the battery (on my trailer there is a 12 v outlet about 4' from the slide switch) and watch the battery power when operating the slide. It should read over 13 volts before you hit the switch. If not the converter is not working. If it does, hit the switch and see how much the voltage drops. If it drops to less than 12 volts you may have a battery that's getting old. Could also be loose connections from Converter to Battery. If the latter is the case your batteries probably take forever to recharge if you run them down a lot.
As for a bad motor, usually if this is the case it will blow the fuse due to excessive current draw.
These are just possibilities if the clean and lube doesn't work.
"A wise man can change his mind. A fool never will."
"Sometimes you're the dog, sometimes you're the tree"
2018 Flagstaff Micro Lite 25BDS
2004 Nissan Titan