From your description of drain tube 'overboard' you have a Dometic fridge
NO it does NOT auto defrost.
As mentioned drain tube comes from drip tray in FOOD compartment and collects/drains any condensation form the FINS
Freezer has NO outside connection.
Ice forms .....time to defrost
That means removing ALL items from freezer. Warm pans of water, hair dryer and towels to collect melting ice.
Don't go stabbing/jabbing/chipping away at the ice....damage to thin freezer walls
If door seals are good and you don't spend time 'lookie-louing' with door open ICE build up should not be overly excessive \\
When we FTd ...defrosting freezer was roughly 3-4 times a YEAR