Hi Yosty,
Welcome to 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.
I don't think I have seen any of the vents come from factory that you can close, but some have installed them. You might start by removing the vents and making sure it is sealed between the duct and vent. I have found on mine that the vent tape came loose and was loosing lots of air.
I'm sure some of out other members will jump in soon with some suggestions also.
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 and your new to you unit.
Jim M