It's not so far out of whack that I'd sweat towing it if it was me... within reason... short trips, hopefully not in the rain... hopefully fixed soon
Honestly it sounds like a very easy adjustment you could make yourself in less than an hour. If you want to bring it in a half inch more than it is, maybe start by adjusting for half that distance first and make sure the slide is operating correctly, not dragging on anything or hosing your floor inside for some reason. If all is good, tweak it a little more.
Frankly, it's going to be impossible for the dealer to know you touched it to begin with. I think that disclaimer was just in case someone does something super stupid with their slideout and ends up dropping it on the asphalt. This isn't one of those cases in my book. I'd go for it.
So, the main thing I'd be looking at is that adjustment that is in the manual looks like it adjusts the *entire* slide, not just the bottom where your issue is. If that is the case, and you adjust the bottom, to close that gap, what happens to the top of the slide?? I would be concerned about any changes you make to the bottom of the slide, impacting the top as well... just take it slow and if the top starts binding, or looks like it's going in too far all of a sudden, back out your changes and take it to the dealer before something is damaged and they make you eat the cost. I don't mean to over think this.. just trying to cover everything I'd be looking at.