My dad has made an excellent storage box which bolts onto your cycle rack on the rear of your motorhome. It is made from 6mm ply to make it very light. It adds a great storage place for anyone who is not using their bike rack and needs some extra space for your outside seats tables etc:
It can take up to the weight of 3 bikes or check out the max weight of your bike rack. If you do make one of these you will need to add the number plate and number plate light for your MOT as shown in the picture. If you do not fit the number plate you could get pulled by the police and fined as it obstructs your vans number plate.