Suggest you find the CAUSE of the problem. Slide locks are NOT the answer if your RV did not come with them (I know of no Montana that did) as original equipment.
We had this issue and our problem was the hydraulic rams which were leaking past interior seals. Not a cheap fix ($1400) but the slides now stay closed and work as designed.
LevelUp, Dual 6 volt batteries, Progressive Industries EMS HW50C, Honda EU2000i Generator, Bridgestone Duravis R250 tires, Torklift Glowstep Revolution Stairs, LED Tail lights
2015 RAM 3500 Laramie SRW LB CC Cummins 6.7L Aisin Trans B&W RVK3600