You can drive a motorhome with a full driving licence if you passed your practical driving test before 1st January 1997 as this gives you category C1 entitlement automatically.
C1 entitlement allows you to drive vehicles up to 7.5 tonnes (7,500kg).
If you passed after 1st January 1997, you will be able to drive any motorhome weighing less than 3.5 tonnes (3,500kg) which includes most modern motorhomes. If you wanted to drive something larger than that, you would have to pass an additional driving test to add the C category to your licence.