I take it that it has only been occasional use for motorhome parking and that you are not registered with any of the organisations such as the Caravan Club who are able to issue a site licence for up to 5 caravans without needing planning permission.
If the above assumptions are right then your landlord is correct only if you have had more than one motorhome overnighting at the same time and on more than 28 nights in any one year. The Caravan Site Act requires all caravan sites to have a site licence.
Schedule 1 of the Act lists the exemptions, basically you do not need a site licence if there is never more than one motorhome and that motorhomes do not overnight for more than 28 days in any calendar year. In planning terms it's known as the "28 day rule". Exceed these limits and your landlord is correct.
You could write and confirm to the landlords solicitor that you have not exceeded these limits and will ensure that you bear them in mind when allowing customers to park overnight.