If your slide is electric, look at the opposite end of the motor from the gears. There should be a lever on the end sticking out of a plastic covering. That is the motor brake. If you have that put the lever in the opposite direction and see if the slide works OK. The lever sets the brake to keep the slide from moving. If your motor has this lever and the slide still moves regardless of the position of the lever, contact me directly rbdtanasi at yahoo and send me the motor number and make. Most likely it is a Power Gear motor. The brake can be replaced by itself but you can't buy it from Power Gear or an RV dealer. I purchased one for my motor from the manufacturer for around $210 delivered to me. The complete motor can be ordered through an RV Dealer from Power Gear and will cost upward of $700.
2001 Monaco Diplomat 40' PDQ - 08 Honda CR-V