Nice touch when they do this.
At the very least, a battery is required for the TT. As already stated, the travel trailers electric brakes need it to operate if it becomes disconnected from the tow vehicle. Any TT that has electric brakes cannot be 'legally' operated on public roads without its own battery.
I don't know of any TT dealership that sell new TT's without providing a battery. The chances are strong that a buyer would not bring their own when they pick it up (TT) to take it home. Not sure what the law is if this were to happen. Can the dealer refuse the owner to leave the site without a battery? If so, could they be held liable if an accident occured on the way home?
2015 Jayco Jay Featherlight SLX16XRB (TT)
2007 Ford F150 Tow Vehicle (TV)