You could have a few problems if you connect power from the vehicle directly to the trailer battery.
If the trailer battery goes dead and you try to operate the hoist you could burn up the connecting wire from the high amperage of the hydraulic pump.
To prevent this you would need to install a battery isolator in the line going to the battery.
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Image courtesy of campingworld.com
Depending on your driving habits a remote battery connected to a vehicles charging system may occasionally not become fully charged.
All electric dump trailers I have come across will rather than be tied to the vehicle charging system, will have an on-board battery charger and be plugged in nightly to charge.