Using a tow bar bike rack or tow bar mounted storage box on a motorhome is generally fine, but may not be suitable for larger motorhomes or US-style RVs. The increased distance from the rear axle in these vehicles can cause too much leverage / force on the tow bar fixings. Additionally, adding a bike rack increases the load on the rear axle while reducing the load on the front axle. It's crucial not to exceed the maximum rear axle load specified on your motorhome's weight plate. Since calculating the additional load from a bike rack or box can be complex (it isn't just a case of adding the tow ball load to the axle load), it's advisable to consult with your tow bar fitter for confirmation.