I would say if truly sewer gas odor (sulferous or raw sewage smell) rather than fungus smell (from wet rotting wood), then most likely IS a plumbing problem.
However, be sure it is not the water smelling - fill a clean gallon jug like an old milk jug with water and smell that - do both cold and hot, because sulferous or iron bacteria in the pipes and/or the hot water tank (or water well) can give off very nasty sulferous gases - and they rise to the highest points in the water pipes when water is not running, so commonly strongest smelling when hot water is first turned on on the top floor if in hot water tank - cold water bacteria smell tends to be more distributed through the piping, and will more commonly produce brownish or orangish water when a faucet is turned on after sitting idle for several days. I would check this possibility out first before chasing possible plumbing issues.
You get the same effect if the plumber used the current lazy mans design with a single vent serving several fixtures, AND connecting downstream of all of them (which pretty much defeats the purpose), AND you then have a clog in the common drain line BEFORE (or upflow) of where the vent line connects to the drain line - so the water entering the pipe displaces the air in the pipe and forces it back up the fixture drains and through the trap.
I am afraid that in my experience, this sort of smell is also commonly an indication of either a leaking drain pipe or a bad toilet seal letting some of the flushed water escape as it enters the drain pipes, causing fungus and sewage saturation in the floor or walls. Easiest way to investigate that is with a fiber-optic inspection camera through the underlying ceiling (takes about 1/2" hole at each inspection point, commonly not more than 2-3 to pretty well checks out an entire bathroom's pipes.
However, if the kitchen and bathroom are far apart (not nearly adjacent rooms) that makes that a bit less likely - though I have seen houses with multiple bad drain pipe connections, or totally missing glue or lead (as applicable) jointing at many joints. Even inspected one brand new house with not a single ABS sewer joint glued - the apprentice following along behind the plumber (who was doing the runs and cuts) thought the purple cleaner was the glue, so religiously applied it to every joint and then stuck them together. Actually went some weeks before leaks started, because with many types the cleaner will slightly bond the joints if you put them together while still wet.
Actually not too easy to check out, because a normal house has a vent pipe leading from the ground level sewer main pipe for your house then multiple vent pipes leading from various drain areas to that riser pipe, so cleaning the main vertical riser pipe does not mean the offshoot vent pipes are also clear. Simplified schematic of typical system here (second to last image, labelled "vent" and "stack") -
http://www.smaller-homes.com/home-plu...
As you can see, there may be multiple fixtures on one vent pipe, and there can even be several vent stacks in a single house, especially in spread-out ranch and mission houses.
You can try running water full flow from a hose down the vent stacks to flush them out - but you need several people running around the house checking fixtures to be sure it is not backing up in any of them, and have the windows open (or be on cellphone) so you can tell the person with the hose to stop putting water into the pipe if it is. And make SURE the pipe you are putting water down is a vent stack - I have seen plumbers and homeowners alike put water down the PVC exhaust ducts from condensing water heaters and furnaces and tankless water heaters.
It could be a vent issue - and unfortunately without some on-the-site investigation one can never tell for sure. However, if that were the case, then when you flush or empty the tub or such, you would almost (there is that cop-out word again) always have bubbling and gurgling or air coming up in one or more of the other drains on the same pipe, or sometimes the same pipe, and only when there is a partial blockage in the pipe.
If only a blocked vent pipe without a partial sewer blockage, what happens is you do not get gases coming up in the basins and tub and such - the draining or flushed water,not having air inflow from the vent pipe to replace the volume of water draining away, suks the water out of the trap and then xxxx air through the drain until the water has flowed away - then the dry trap lets sewer gases come in. Therefore, if when flushing or draining the tub, say (a large quantity of water), if the water in the bottom of the toilet gets sucked out and gurgles as air goes past it into the pipe, and air is sucked into the basins or tub/shower (test with a large piece of saran wrap over the drain - and hold onto or weight down so it does not get sucked into the trap), then your issue is a lack of makeup air from the vent system. On the other hand, if after flushing or starting to empty tub, air starts coming up into the toilet bowl (causing visible rising air bubbles) and the bowl actually fills up a bit from the trap and air comes out of the tub/sink drains causing the saran wrap to balloon up, you have a partial blockage causing the water to displace the air in the drain pipe and it is rising up into the drains. Give it the test and see what happens - might be a useful exercise for you. If the air is coming up out of the drains, then full-diameter routing (usually by a sewer and drain cleaner, not a plumber unless a large outfit that has the big drain cleaner machine) is called for - and chase it all the way to the street so the clog does not just get hung up again somewhere further down the pipe and cause the same problem again in a few days or weeks.
If not specifically located, I would make sure (If you have not already) that the smell is coming from the drains and not the surrounding walls or floors. Once you have the drains tied down as the cause, I would get the entire system routed out with scraper blade, which cleans full diameter, not just punches a hole through clogs - which should be done every 10-20 years depending on how fast soap scum and grease build up in your pipes. Have all the stacks run too. Then I would give it a week to see if it clears up. If not, then have the sewer and drain cleaner guy come back and run a camera to see it he can see any clogs.
