We visited a weighbridge when fully loaded with a 125cc 120kg scooter including full tank of fuel on a 40kg rack on the back, with a full tank of fresh water, full tank of diesel, both passengers seated, 2 ebikes in the garage (yes I know!) and kitted out with food etc for a foreign trip.
The total payload was 690kg over and beyond the mass in running order MIRO which, for our vehicle, was within its payload limit. Both axles were within their load limits of 2500kg at the rear and 2000kg at the front with 42%
of the weight on the front axle. We have a 4250kg motorhome and have enough spare payload to not have to consider any weight saving options.
There are very few, if any motorhomes, in the under 3500kg range, that have the legal limits to carry a scooter or motorbike on the back without seriously prejudicing the ability to carry anything else!
However if you do have a 3500kg motorhome and you have an empty water tank, a half full fuel tank, and leave the wife at home, you should be OK!