If you have a trailer that weighs x pounds, what kind of margin above that would you consider to safe when driving? What I mean is, I will be buying another minivan sometime in the next couple of years and they all have a tow rating of 3500lb or so. Most of the smaller trailer seem to weigh in just below that. Is that buffer of a few hundred pounds enough of a safety margin or should I really just pitch a tent?
You'll need a margin of way more than a few hundred pounds.
The quoted empty weight of the trailer is usually the absolute bare minimum and doesn't include any "options" like propane tanks, air conditioning, microwave, awning, bbq, etc. The real empty weight of the trailer could easily be a few hundred pounds more than what the brochure says.
And then you have to add the weight of all your stuff - dishes, cutlery, pots, pans, groceries, coolers, camping chairs, clothes, etc. It can add up to way more than you'd expect.
And don't forget that your minivan's 3500 lb towing capacity is a best-case scenario, and can usually only be achieved with no other passengers in the van except the driver. Any passengers and cargo you add will reduce the available payload, which means you will have to reduce the trailer tongue weight, which means you will have to reduce the trailer weight. Once your family and gear is in the minivan, I wouldn't be surprised if the actual trailer weight limit isn't closer to 1,500 lbs. And if you load enough gear into the van, your trailer limit could easily be zero! You'll need to get the payload and towing specs for your minivan and do some math to figure it out.
And wind resistance is a big issue, too. I'd be looking at only tent trailers or the narrower and more aerodynamic models like rrocket has shown. Even if it's light enough, the wind resistance of a traditional 8' wide by 10' high trailer is going to work a minivan really hard.
