Sluggish & Overflowing toilets
It's interesting that this problem seems to have developed in the past year. This suggests that the vent pipes were properly installed.
Also...regarding the sewer smell in the washer: Does the washer drain hose go UP to washer height then drain DOWN into a standpipe or tub? If so, this is a GOOD thing, as it prevents backflow (accidental siphoning from drainpipes into the washer). Also if so, it's unlikely you're getting waste water into the washer (hopefully you're not)--unless waste water is getting into the fresh water supply elsewhere--and we really don't want to "go there", do we...
Also, washer hose goes up into PVC then down then into a bigger PVC then out the cement wall. I've never noticed any wetness in the washer, just the smell, which is why I thought the toilet problems may be coming from the vent. It seems if it is blocked the fumes maybe going into the washer seeing the septic is so close to the washer. I just thought the toilet problems and the fumes from the washer pointed the problem being the vent.
If it is the vent and we can't get to it for another month or 2, is there any temporary solution??? Plunging it seems to work for a short time, although they are still sluggish.