Hi kdlokken,
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 think we need a little more info to be able to give good suggestions, when, where, all the time...kind of unit? It's not uncommon to have smells if your hooked up and leave your gray tanks open. You would get gas from park system. The vent/check under the sink may not be working correctly. Also maybe the vent from the black tank maybe venting into the attic space due to vent pipe being loose or something.
Let us know a little more info and if you find the problem.
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