Toilet needs hisit's own vent pipe, you. You see them on the roofs coming out, those. They are the vents forfor drain pipes.
Install one for your toilet.
What you call a sewer vent cap, is actually where you connect the drain vent pipe up truethrough the roof.
Since it is under the sink it is also the vent for the sink.
We call that incomplete installation.
If you do not like drilling holes in your roof, you can have it exit truethrough the wall. Cover it with mosquito net to stop Lake monsters coming in. Then put a nice vent cap (vented) on it to make it look good.