I moved into my parents newer modular home almost 2 months ago. They are only here 6 times a year.?

The house just started smelling like the septic system. One bathroom smells so bad (like #2) that I put a rolled towel down to keep the smell in. (Not working) The other bathroom smells like #1. The system is only 2 years old. Never had issues, never been pumped. I called the guy who installed it, he said he wouldn't come out because it wasn't backing up. I have read suggestions to climb on the roof, maybe a bird built a nest in the vent? Is that likely? Is there something I should (or should not) flush down to help? Help! It smells so bad I don't know what to do? What else could it be?