Plumbing backing up issues

Hello,

I own a duplex where I live in one side and rent out the other side. We had issues last year with the mainline getting clogged causing gurgling sounds in the kitchen sinks, slow draining bathroom sinks, etc. A plumber came and ran a snake thru my downstairs toilet which solved the problem, though he did say the mainline is much older than the plumbing in the house and will eventually need to be replaced because of an issue with there not being enough fall from the newer house plumbing to the line. He ran a camera, found the line dug it up and put in a clean out.

Now my tenants are saying that both their downstairs and upstairs bathroom sinks are very slow to drain. There hasn't been any gurgling and all the sinks on my side are draining fine as far as I can tell. However, they noticed a nasty residue in their downstairs shower indicating something must have backed up into it and drained back down. There was a hint of a bad smell in the shower when I poked my head in the shower...and they normally keep a very clean house so I'm sure it isn't from lack of cleaning.

Any advice on what to do? Should I rent a snake, access the clean out and snake towards the house from the backyard which would be where trouble spot with the mainline/home plumbing transition? I'd like to avoid spending the multiple thousands that it will cost to dig up my yard, move a deck and replace the mainline...at least for as long as possible.

Thanks!

Normally, gurgling sounds don't come from the lack of draining they come from vapor lock because the vent stack is clogged somewhere and air can no longer get out to break a pressure condition caused by draining action. The gurgling comes from sewage gas breaking through the water and coming out the drain. The room will normally have a sewage oder when gurgling occurs.

It's possible in your house one of the drain lines was plugged up enough to cause the liquid level to rise high enough that the vent stack could no longer function. If the plumbing is laid out correctly this would never happen.
You might get slow draining but you should never get gurgling.

The sewage lines on duplex are supposed to be completely separate from each other. The only time it wouldn't be is if the house is really old and was later split before this became the normal plumbing code.

You need to look at how those sinks drain and where the tub that backed up drains to. It may be a smaller line that is just somewhat blocked.

Fill up the tub or a sink having the trouble until it starts to fill the sink leaving the drain open. Then flush the toilet about a dozen times on the main floor. If the toilet always goes down quick but the sink doesn't then the problem is likely in the smaller line and not the main line.

If both get blocked up eventually with enough water draining then the problem is in the main line. Start by renting the big snake. You'll need to open the sewage hatch either in the basement or the crawlspace depending and snake going away from the house.

If you can't fix it with that then you are going to need a plumber to stick in a borescope to see what's going on. It's possible it's plugged with a tampon/pad or something else silly that shouldn't be there or the main line has collapsed or been breached by tree roots, etc. etc.