Battery boxes on pop-ups are almost always outside. The tongue is the usual place. The boxes are not permanently mounted to the tongue. They can be lost when a previous owner does not finish a repair job or when they are stolen. A new trailer may be shipped without a box or it may have been lost during shipping.
Look for heavy cables near the front that are hanging loose or wound into the structure. Where does the tow connector cable go to? Follow the cable.
If the pop-up does not have electric breaks, it may never have had a battery.
Post pictures of the front area. Some one may spot a clue for you. Also post the make and model. It may be on a sticker on the left front of the trailer.
I wish you good luck and happy trails ahead!