Increasing the flooring thickness is risky, at least to the flooring (scratching or tearing) and possibly even to the slide if it's high enough.
You didn't mention the coach brand and year, so I can't guess what sort of slide mechanism you have. Above-floor slides are typically no problem as long as the height isn't extreme, but flush floor slides ride very close to the floor. Some flush-floor slides have rollers underneath, others may have skid pads, and some have nothing and ride only on the lip of the sidewall. All are susceptible to scraping the floor or leaving tracks behind. If only a bit tight the rollers or pads may just leave dirt tracks, but over time there is likely to be wear marks as well. If really tight, it will scratch and gouge. For the slides without pads or rollers, sometimes the angle of the slide can be adjusted slightly to give a bit more clearance inside as it comes in. This has a slight effect when fully extended too, but should not be noticeable.
You need to get down at floor level and watch the underside of the slide carefully. Use a strong light to hep see, and maybe slide a thin piece of material under to check for tight spots. An inspection camera helps. http://www.harborfreight.com/digital...era-67979.html
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL