If you are talking about a slide out... I have heard of a few coach converters that did. I posted a link
to a guy who built a fiver with a slide out (scroll down to the bottom of the page). You need to make sure you beef up the framing all around the opening with cross bracing. and then you will need to deal with all the problems that slide outs develop (leaks, mechanisms messing up, the slide "opening" while traveling down the road, water/debris/ice on the roof of the slide out has to be removed before closing, in cold weather you either find a way to seal the cold out or you keep the slide pulled in as they are HUGE heat sinks...etc). If you must have a slide.. get a slide topper to go with it. It will save you cleaning time every time you close the slide (you have to climb up and make sure there is no debris or critters on the roof of the slide out and under the slide topper). All so that you can gain less than 24" of space. And the mechanism for the slide outs are very pricey. Adding a "hole in the wall" is asking for trouble in my opinion. I've read too many of the RVer's having mega problems with slides. Of course if you have a stressed skin bus, forget it. That's why you don't see many MCIs with a slide out. Stressed skins are integral part of the frame integrity. You don't cut huge holes in the sides without almost rebuilding the frame of the whole bus (floor, roof and opposite wall have to be reenforced along with the wall you cut the hole out).