A modification can be a simple cosmetic change like new fabric for the seat cushions, or a floor plan change like a re-arrangement of the camper dinette, or the addition of a new or better component or appliance, and anything in between. Interior and exterior truck camper modifications and decorations count.
The best modifications are things you did to improve the aesthetics or function of the camper to better suit your personal needs. Did you alter your camper to make it work better for your truck camping lifestyle? Chances are you made a qualifying modification.
Good luck mod makers!