Trailer wiring help

We bought a Pierce Carriage car trailer. It is a hydraulically controlled sliding axle trailer that doesn't need ramps because the tail end of the trailer drops as the axles are pulled forward. It has it's own hydraulic pump and battery. The literature that we got with it had us tie our tow vehicle battery source to the trailer to keep our trailer battery charged. It has worked fine for us for years. If the trailer sits for months at a time we still have to recharge it. But once charged the truck will maintain the trailer battery. When we've used it behind vehicles without the extra 'charging' wire we tend to have insufficient power when we need it.