adwsystems:
You can get a buck converter to drop the voltage. The problem will be handling the amperage. The brake bulbs on my trailer are 55W each. 110W total divided by 12 gives almost 10 amps. Which I think you will be hard pressed to find one that can handle that.I would look at using a battery box to improve the packaging on the current system.
Its much worse than that, a cold filament draws nearly 10 times the current of a hot one in a tungsten
bulb, so 55W 12V bulb will pull 40A or so peak.
You can either use an expensive current limited supply (the bulb will heat up anyway, just a bit slower), or
go back to using 12V battery (3S LiPo pack might be a possibility).
[ Most buck converters will not handle current overload gracefully, they will just shutdown to avoid
inductor saturation (which would put the input voltage directly on the output). ]