I would assume a good battery is a gift. Plan on replacing within one year.
A 2-way fridge uses 12v for controls and either propane or utility power to power the absorption process of making the fridge cold. If there was an inverter that would be to power a residential compressor fridge or for general use.
Most do not have an OEM inverter so that is something the owner added. Be wary of add-ons. Some are jacked into place.
All do have a converter for charging the battery when plugged in.
All trailers have a provision for a battery but if it has been stationary for a while the battery may have been removed by the owner.
The 12 Volt Side of Life