On ours and every Montana I have seen, there are bulb seals on the trim around the slides that seal to the side walls of the camper when the slides are closed.
There is also another set of bulb seals (different ) that are adhered to the interior walls (sides and top) of the camper at each slide that seal to the back side of the slide out interior trim when the slides are out.
Are any of these seals completely air tight ? NO, but there is not much that can be done to completely seal up a slide out. It is what it is.
2018 Chevy 3500 LTZ Dually Diesel 4x4 CCLB
2011 Montana 3455 SA. 6 point level up. Disc brakes. Curt Q24 Hitch
Progressive EMS. Valterra tank valves. Sailun G637 tires. ARP fridge control. All led lighting. Mor Ryde IS