Motorhome rear box fitting

Having assessed the weight of the motorhome we want I am now considering the fitting of a Thule Carry All box or similar as the motorhome we want has no garage.  Additional storage will be preferable to buying a larger or different model.  

Most of the boxes available appear to be designed to fit a bike carrier.  We neither want nor need a bike carrier.  Furthermore, the Thule web site states that their box can be attached using a simple bracket rather than a bike carrier.  I suspect the latter, which we don't want, is more expensive than a bracket anyway.  Can anyone please tell me if the fitting of such a bracket is a simple process or otherwise?  I know that an obvious answer is "Ask your dealer". However we have not yet decided on a dealer as we are still in the preliminary stages of the process.