Having a bike rack fitted to the rear of your 'van is commonplace these days but I have also seen quite a few motor scooter racks fitted, and not all of them look the safest of installations. For one thing, few motorhomes have a back panel that is sufficiently reinforced to carry that kind of weight. Even if the panel is strong enough, there can be other problems concerned with the weight of a scooter, even if the overall payload limits are not being breached.
Fitting a scooter rack to the rear of a motorhome is something that should only be done after you have checked your axle loadings. Only then can you consider what effect the weight of the scooter, and its mounting rack, will have on your rear axle loading.

The calculation may seem a bit daunting but using this basic formula it is a reasonably straightforward matter:
A1 = existing front axle load (before adding on the new rear load)
A2 = existing rear axle load (before adding on the new rear load)
L = weight of new rear load
W = wheelbase (the distance between the front and rear axle centre-lines)
O = overhang (the distance from the centre of the rear axle to the centre of mass of the new rear load).
Add the figure for the new rear load, and you will be able to carry out the following calculations:
New front axle load (new A1) = A1 – [L x (O ÷ W)]
New rear axle load (new A2) = A2 + L + (A1 – new A1).
For example, take a 'van with a current front axle load (A1) of 1500kg and a rear axle load (A2) of 2000kg (see diagram). The scooter and its rack have a total mass of 150kg, the vehicle's wheelbase (W) is 3000mm and the overhang (O) is 1000mm. Put those figures into the above equations and the new front axle load will be: 1500kg – [150 x (1000 ÷ 3000)]. This works out at 1500 – 49.95 which is a new front axle load of 1450.05kg.
Now that we have a new A1 figure, we can move on to the second calculation to get the new rear axle load: 2000 + 150 + (1500 – 1450.05) is a new load of 2199.95kg. In other words, the scooter and rack would add nearly 200kg to the back axle, while taking almost 50kg off the front axle.
You will need to compare these figures to the maximum axle loadings shown on the vehicle plate. If your new rear axle load is greater than that on the plate, then you can't fit the rack and motor scooter.