Even if you do not personally have a copy of the policy if its arranged through Mobility, the scheme and the features it includes or excludes should be fully explained in the mobility literature you see and sign.
Mobility have a standard breakdown policy for the vehicle only and not the caravan. If you need the caravan recovered, generally you need to make your own arrangements whether that is through Motablity or not.
When I had a Motability car, I still had Mayday cover for which I paid in addition to the RAC breakdown cover. You did not get a copy of the breakdown policy as Motability were leasing on your behalf and then leasing it again to the customer.
They did not explain anything in the documentation about breakdown cover except to say that only the motor vehicle was covered for breakdown as the majority of people with a Motability vehicle would not be towing a caravan.