I was looking at a used Artic Fox camper and requested to load it on my truck and drive it around to see how my truck handled it. I was flat out denied and at the time was extremely upset(I'd been looking for this model for several weeks). Left right then without purchasing and in hindsight, glad I was denied as I purchased my Bigfoot and couldn't be happier.
You can thank lawyers and McDonald's overly hot coffee for our society being as litigious as it is these days. Liability, liability, liability... it sucks beyond comprehension just how much society has begun to accept the nanny state of forced compliance to blanket policies because of "liability". Don't blame the dealerships, the camper isn't paid for and should you get in a wreck on your short drive around the block with their camper they are now out of pocket.
Private party you might actually have better luck with such a request because as someone already said, everything's negotiable (unless it's the blanket policy of a corporation or the government...).
For any future readers that might dig out this thread while researching buying a camper (and at the risk of starting another weight police flame war), here's a thought I will leave you with. If have never driven with a camper on your truck before then when you are shopping for a camper, buy one that is well within the weight limits of your truck and drive knowing you that you are within the manufacturers designed tolerances. On the other hand, if you have experience on the road with a camper and know what it generally "feels" like then pushing the limits of your trucks GVWR might be more like "guidelines" rather than a hard rule.
Taking this to a dealership that won't let you test drive... If you've never driven with a camper before and aren't certain about how your rig will handle and they won't let you test drive it then make certain you are within the GVWR for peace of mind. Or, walk away and find another camper.