Firstly you need a deep cycle battery otherwise known as a leisure battery as it is constructed differently to that of a shallow cycle battery used for vehicles. You also need to measure the width and height of the battery box to make sure that a battery you buy is not too big and allows clearance on the top for the battery connectors. The size in amp hours all depends on the demand such as a water pump, electric flush toilet, motor mover, TV and anything else that relies on 12 volt power. The greater amp hours the battery the more hours it will provide for example, in theory a 110 ah. battery will provide a further 25 hours than an 85 ah. battery. If you are on a mains electric hook up then the amp hours is not so critical as the onboard charger (if working effectively) will replenish the power consumed.
If the FC has a battery box compartment with a lock up drop down door then there should be an onboard charger which is normally located directly behind the battery compartment under a seat or in a cupboard. The charger otherwise know as a transformer will be either a white or silver box with vents on the cover with two cables coming out.
Have a look at the link below which is posted for illustration purposes only but will give you an idea what the charger/transformer looks like.
Is there a round 3 pin socket with a flip up lid on the exterior to connect an electric hook up mains cable to identical to the one shown in the link?
If there isn't then the FC is only reliant on 12 volt power (battery) in which case there won't be an onboard transformer.
You could probably install an onboard transformer but as I'm not a qualified electrician I couldn't advise how to connect it up to the 12 volt circuit and mains MCB trip switches. A service engineer would be the best person to ask as they could give a price on the parts required and labour costs but it won't be cheap as the price of a charger/transformer is around 100 quid alone.