There are several threads (around the same time, 2016) that ask the same question or complain about the cupboard doors near the slide not fully opening. It's not a common complaint, but other members have commented about the same type condition. Some have found that adjusting the slide extension gives them a small bit better sealing around the slide. Others have found that their slide is already at "full extension" and can't be adjusted so the flange is closer to the sidewall of the trailer.
I'd suggest you first use a mirror and flashlight to actually see if the seals on the slide flanges (top, front and rear flanges) are compressed against the inside wall on the trailer sidewall. If they are, then the slide is at maximum extension and your dealer is correct, it can't be adjusted to extend further.
If, on the other hand, when you look at the slide sidewall, you see a gap between the gasket and the trailer sidewall, then the slide "should extend further" until the gasket is compressed slightly and completely seals the trailer interior from the outside air infiltration.
DON'T ATTEMPT TO ADJUST THE SLIDE USING CUPBOARD CLEARANCE AS A GUIDE... The slide flange clearance/seal compression against the trailer sidewall should be your "stopping point". Don't try to overextend your slide, you may wind up damaging the trailer.
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS