nearly all travel trailers i've seen, have the battery/batteries on the tongue.
sounds like the seller didn't provide one. why was that? did you not check the 12v system before you bought it?
did you not go thru the trailer to make sure everything worked?
if the seller let you leave without a battery, you could have been ticketed for pulling a TT without a battery to power the emergency brake system.
by the way, welcome to the "Roo Kroo"!!!
be sure to post future question and pics at the Expandable/Hybrid section here at FRF. the Roo is not a Tent Camper, it's a Hybrid.
lots of Roo owners there.