If you can manage a hull wash after three weeks it will just about be OK
Antifouling will devalue the RIB and be a load of work
If it has been previously antifouled, and washing is a hassle, then a quick lick of an ablative like Cruiser Uno will get you away. Advice from the area you are mooring in would be good - not all antifouls do well in certain localities.
Entirely bored with Coronavirus now...