If you've already got some experience towing utility trailers, you are ahead of many other people. Some things I always mention about trailers, which you might already know:
Get used to using your side mirrors on your truck and other vehicles. Don't use the center windshield mirror when you are just driving around, towing or not. Learn what the side mirrors are showing you.
If your helper is not used to giving trailer backing directions, their primary job then is to warn if you are going to hit anything.
Your "helper"... have them tell you which direction the rear of the trailer should go, NOT which way they think you should turn your truck's front wheels.
As others have posted, the helper should stay within sight of the driver's mirror, preferably on the driver's side. The helper should understand that as the truck turns, the view out of the driver's mirrors changes, and the helper should move to stay within sight. If the helper needs to move to see another part of the trailer, stop backing up and wait for the helper to come within sight again. Your helper does not have to be right next to the trailer, they can stand pretty far away to see what they need to see and still be within sight of the driver.
Get out and look! If in doubt, just stop, get out and look. If you have to get out a dozen times to look, then get out a dozen times. The more you do it, the less you'll need to stop and get out.
Try not to let neighboring campers watching you bother you - we've ALL gone through it, they are not laughing AT you, but laughing (or smiling) at remembering their OWN backing up adventures!
If you have to temporarily block the road to back into your site -- EVERYONE has done this, and has to do this to back in. If anyone drives up and is waiting to get by you, don't worry about it. They will patiently wait. Maybe even smile and wave at you, they are not making fun of you!
Have fun camping, and see you on the road!