Unfortunately, you are only reserving a size site instead of a specific site. Not sure how Canyon is since I've never stayed there, but if it was down at Bridge Bay, I wouldn't be too worried and make the reservation. Odds are, you will be able to get the rig in the site and out of the way. Worse case, you can confirm with xantarra if you want, you may have to park your truck down in the alternate parking lot, just as if you had a second vehicle.
Remember, one thing they don't count on is being able to let the rear 10-15' of the trailer hang over the rear of the campsite. Cannot count on this all the time, but usually you can get a few feet at least. You only need 7 and could squeeze that down to 5 by backing under a little extra.
I think Canyon is a bit tighter than Bridge Bay because of the heavier tree coverage.
But yes, there is a risk at reserving the site.
