Question Chris_p, are you going to use the retaining wall to raise up the soil so the ground is level to the top of the slab or to lower the soil and have the retaining wall hold back the soil that is level to the foundation. Sorry my pea brain is having a hard time understanding your vision. I would guess you want to come out with a wall and then lower the ground in front of the wall to about the same level as the trailer. Digging that with all the types of fill could be some tough sledding, If the room permits, I think I would build the wall out 3-4 feet away from the foundation to the location you can dig down about a foot for gravel for the base of the wall avoiding the fill. Then back fill behind that new wall level to the top of the foundation. A metal probe made out of #3 rebar welded into a tee would allow you to define the location of the edge of the fill by pushing it down into the ground to detect the fill along the wall path.
Hope this helps