I think you have some likely hood of damaging your van's transmission. A 25 foot trailer, I assume camping trailer, would weigh at least 4000 lbs empty plus with supplies and equipment another 1000, assuming you don't fill the water tank. Plus the trailer will add a lot of wind drag. Something else that shouldn't be lost is that making it up the hills is hard but sometimes after you've gotten your brakes hot coming down is a whole lot worse. I travel a lot and have seen some pretty bad wrecks when trailers are involved. If you must do this, drive slow and never use overdrive and be very mindful of your stopping distance. One quick point about your parents car if it was an old sedan from the 70s it would have weighed 1500 lbs more than your van and would probably had a large V8 engine and lower gearing than your van.