Pluming Question - Septic

[Gas Sucks]

I bought a trailer house 2 years ago while i save money to build a house with cash, me and my dad installed septic tank, every since i moved every once in a while my bathroom will have a strong methane gas odor, it is coming from vents under the sink, the vents have brand new check valve on them, a while ago i covered them with wrap and that stopped the smell for a while but now it is back. i know you are not supposed to cover them but it is the only thing i could do to get smell to go away. I called a plumber about 8 months ago to check it out and he changed the wax ring on my toilet without asking me 1st and charged me $125 for a $1 part and said that was my problem even though i knew that wasnt what it was. any ideas on how to solve this