Here's a cheap, easy and reliable electronic aide, buy an object detector like we have on the bumper of the DWs Explorer. They are about $15 on eBay and have 4 little sensors and the control unit and a visual/audio display for the dash.
Because you won't be going 88 feet per second while entering a campsite, if you place one near the rear of the highest object and one slightly in front, you will be warned in plenty of time to stop. The sensors will start beeping at approximately one yard and the beeping increases in frequency until you are about a foot away from the object and goes to a continuous tone.
I would guess that most of us pull into a site at about the same speed as we parallel park our toweds, this should give you plenty of time to stop and look before pulling in the site.
My AC shroud is curved enough that I could attach one at about 45 degrees from vertical. My towed doesn't have a system so I bought one and when it comes back from the body shop I will install it.
If your in need on the open road, I think Dennis(Executive) is correct.