Fender well storage box?

My previous camper had doors behind and in front of the wheel on the driverside that had access through fairly large inside doors. I used the large square plastic boxes like kitty litter comes in, probably around 5 Gal. They had watertight lid (if you didn't submerge them). I would put one forward and one back plus other items that didn't need to be kept dry. The new camper has access to both sides in the front from inside the camper but only the drivers side is big enough for these plastic containers to fit through so I put two in the front plus extra stuff and smaller things on the passenger side front that don't have to be kept dry. The back is accessible from outside doors in the back of the camper but they are narrow and I haven't found the best way to use that space yet. I did make some free standing racks so I could haul long poles or our awning on the passenger side, over the top of the wheelwell.