Don't worry about it, the site measurements aren't that precise - an extra 2' certainly isn't going to leave your trailer sticking out onto the road. Also, on 99% of sites, parking for your TV is on the same pad, so there's almost always lots of room. If you do happen on the 1%, you should be able to angle your TV slightly so that everything fits, or - absolute worse case scenario (which I honestly can't imagine happening) - you can just park your TV in the overflow lot.
Now, I wouldn't recommend trying to squeeze a 30' trailer on an 18' site without foreknowledge of the specific site... but you'll be just fine.