I would get a large plastic tote, dig a hole going into it, run the plumbing to it, and bury the whole thing leaving a cleanout for pumping. Talk to someone about pumping it on a regular basis to find out the best way to set it up for access.
You will also have to insulate the line going from your black tank to the big tank, or it will freeze up in the winter. Do not leave the black tank open all the time to the big tank or you will get a big mound of poo that will plug up the black tank. Just dump it when it's close to full.
Truthfully for the gray water, just hold your use down and run it out a hose about 100 feet from the trailer. Beware, this may freeze in the winter too.