I am not sure if any one has ever come across this problem, I'm not sure what I need to do and thought maybe someone would be able to help me.

I had a modular home built last April with a basement the land I own is in the part of the town where some have regular sewers and some have septic, I happen to be in the area where it's septic. We had a real old house on the property with a newer septic system, but because we had a bigger home built we weren't able to use the 5 year old system and had to buy a new one. When we bought the home we opted to do all the finish work which included the septic. Every thing on the outside of the home with the septic was done right and approved. Now on to the problem, when the manufacture of this home put the home on our property he did not inform us about the vents that needed to be put in the roof. I have a real good nose for smells, I guess it's because I have a lot of sinus & allergy problems and had to have 3 sinus surgeries which open them up but maybe not for the better. I noticed this funny smell right away which caused us to look into where it was coming. That was when we realized the house was never vented. Next step, we vented every place in the attic we could locate and cap them off outside, hoping that would solve the problem. The bad smell coming from the bathrooms did go away in two of the bathrooms, the third when you turn the water on it still has a bad smell. Next problem, in one of the bedrooms where the attic is there is still a smell, I try to describe it and say it smells like a dusty room, or the smell you get when you go into the attic from the insolation. Some days it's really bad and sets my allergies off, I now have a sinus infection for almost 2 weeks, not even the medicine I was to take will clear it up. I am also on prescription allergy pills. As you might be able to tell I am at a loss and need to find out how to stop this smell from coming into the house. It is mostly up stairs in the room where the attic is and the adjoining bedrooms also have a slight smell.
Someone told us that it could be the gases coming into the house from the septic tank but I am not sure if that can happen or how else to correct it. I'm hoping that maybe someone could help answer my question; it would be greatly appreciated. THANKS~ Kat