Little Door into Truck Box Front Left Question

Put a bumper mount to hold it (mounted under the overhang if you have one or along the truck bumper), put it along the side (if you have a shorter camper some have doors at the back along the side for things like this or put a mount above the box and below the camper).

The bag will get soiled sooner or later and will leave "things" on your floor, door, etc. Dump areas are not known for being the most sanitary places.