Issue Backing into Camping Spot

Last year was our first year with our camper (27ft. Heartland Trail Runner). Although we didn't have any major issues that weren't easily corrected, I found that I struggled in one area and likely will again this upcoming season. When I pull past our campsite upon arrival, I can't judge when I've gone far enough so I can stop and then begin backing in. I can't see well enough with my mirrors to know for sure where the back of my camper is in relation to the campsite entry and of course the camper blocks your view. I either pull too far past and have to back up some before starting to turn and back in, or I haven't gone far enough. I end up having to get out of my truck, walk to the back of my camper to determine if I'm in the right spot to begin backing. I love my wife dearly, but she is no help whatsoever for this task so it's all on me. If I could back in where I could look out the drivers side window, I wouldn't have nearly as much trouble but every campground we've been to have you back in like in the picture below. Is there a general rule of thumb or something you have learned to determine when you're in the right spot to begin backing and turning into your campsite? thanks in advance for your expertise!