Trailer Battery Charging System

The trailer wheel is not a power source. Instead, it will become a drag on the towing vehicle's engine, which will be the real power source.

If you replace the trailer axle with one from a vehicle with a differential, then you could drive a generator from the axle. Not knowing the size and configuration of the trailer in question means I can only give the barest of information which will require your imagination to fill in the blanks.

Drag will be dependant on wheel size, gearing and the size of the generator.

36Volt is not a usual automotive (motorbike/car/truck/boat) voltage otherwise you could use a feed from the trailer wiring to maintain the battery charge.

You could fit a small petrol/diesel motor running the 36volt generator onto the drawbar of the trailer (if it is an "A" frame design).

If it is an enclosed trailer then you could mount appropriately rated solar panels into the roof of the trailer to provide charging of the batteries whether you are mobile or parked as long as there is available sunshine.

You need to provide more salient information in order that you can be given a more precise and considered response.