I do not know if you plan to do this but this is what I would do. Probably giving more basic direction and if you have a plan feel free to ignore me(I am used to it - wife does it all the time)

1. drill pocket holes in counter - 1 every 6"-8" is plenty
2. clamp a wide board against the side of your trailer that will support the counter at the height you want the counter on each side. Board should go from floor to counter height
3. Screw counter to sides and remove boards
One of the nice things about doing it this way is that now you have the height of your cabinets already measured(you can measure your support pieces and cut the sides of your under-counter cabnets and your counter can act as the top of your cabinets.