Hi theramblinrv,
Welcome to the posting side of the Heartland Owners Forum and to the family. We have a great bunch of folks here with lots of information and all willing to share their knowledge when needed.
A lot of the time they put a sticker or decal on the wall in the aprox place to drill for the vent. You can use a stud finder also to locate the frame work. And most folks use a small drill to check for clearance before making the big cut. If you don't get the answer soon from other members or reply from the factory, I suggest you call Heartland Customer Service, 574-262-8030 or 877-262-8032, have your VIN when you call.
Be sure and check out our Heartland Owners Club. Join us at a rally when you can and meet lots of the great folks here and make friends for a lifetime.
Enjoy the forum.
Jim M