Can sewage smell be from water heater if water DOES NOT smell?
Since you said your gas lines are copper I would guess you have a gas water heater.
If your water heater is not properly vented AND has a sufficient source of fresh air this is probably the source of the problem.
What is happening? The most common problem is insufficient make up air. We know the gas water heater requires a vent and I have never run into a no vent problem.
The cause I have found quite a few times is not enough fresh air so the exhaust vent takes the air out but then the home goes slightly negative as far as pressure.
When this happens the room air is consumed and the exhaust gasses circulate in the air space leaving the foul odor you have when the water heater is running for a while.
To prove this open a window in the basement a bit if there is one or make sure there is a fresh air source to the basement to verify this would fix the problem.
To make a permanent fix a fresh air source should be ducted to the water heater area and in most areas this is required by code.
Give a fresh air source a try and it will probably solve your smell problem.