I ran 100 amp service out to my garage when it was built. I just bought 100A direct burial "trailer feed" wire at Home Depot, rented a trencher and put it 36" underground.
You might also be able to simply use a knife trencher that pulls the wire underground as it moves along then closes the trench back up - all in one pass.
I couldn't do this in my case because there were pipes and tree roots I had to negotiate.
For a camper feed, you can just use 6ga direct burial cable since you only need about 30A feed.
This is how it's done:
http://www.youtube.com/watch?v=FUQKIMqM5dw