Upgrading the trailer is going to be way easier no matter how you look at it.
If you go with larger discs or drums on your van then you most likely will need larger wheels to clear the brakes and possibly a larger master cylinder to deal with the larger brake components. Pricey
For the trailer if you swap a larger used axle entirely from a used trailer off craigslist or go to your local trailer place and get components or even a new axle.
I plan on doing this with mine, I have a 8X12 that I pull with my 3500 silverado. I dont think i could load teh trailer up with enough weight to mess with teh truck without the trailer wheels popping
But I would want the ability to put 1 bike on the trailer and tow it with my wifes Equinox in an emergency. So I plan to swap teh axle or get brakes in teh spring.