When you say slider, assume you are talking about a slideout, not a sliding hitch or....
The slide seals only seal when fully closed or fully extended.
I don't really think you will run into ANY campground where that little bit of slide should cause you concern.... if it's THAT tight, I wouldn't want to stay there, your entry door may hit the next rv
but if you are talking about just tailgating,
My guess is that most parking spots are 9'Wx18'L and guessing your rv's slide being out would not be 9' ???
if so, then yes, you can do whatever, just knowing rain, bugs, heat, cold will/could get by the seals.