13.88 is a good indication that the caravans low voltage dcc power supply is working. Good.
The Fanmaster was designed to do three things.
To blow unheated air
To blow air heated by the gas heater
To blow air heated by a 230V ac element
Because the gas heater is based on a design used before mains power was widely used in caravans, only low voltage 12V dc power was available to run a fan, so the Fanmaster's fan needs an external 12V supply.
The Fanmaster only uses 230V to heat a set of mains heating elements built into the casting.
The Fanmasters control circuit needs 12V dc to run, and it controls the current supplied to the fan motor to control its speed. The control circuit also regulates the 230V current to the heating elements depending on how the control switch is set.
Certainly if the motor is faulty you might see 12V power at the motor terminals, But the absence of power at the motor terminals, could be fault on the main circuit board, or wiring feeding the Fanmaster.
If you do have good access to the behind the heater, with appropriate safety measures it's not too difficult to remove the Fanmaster from the back of the heater and its ducting. Getting it out will allow you check it out more easily. It will almost certainly benefit from a careful clean to remove dust. Be careful not to remove any balancing clips or bend any part of the fan impeller when cleaning.