Ok, so I see this might have been a design flaw in the original build so there isn't much you can do unless you rip it all out.
You could have it all scoped and see what's what so you can get an idea of what you are dealing with.
You could comp the guests a night. That isn't a bad way to keep folks happy and make sure they want to leave a 5 star review.
Yeah, the toilet is one thing but the tub, whew. Nasty! :)
I am sure this is a solvable issue with some work.
Do you happen to have an HOA covering the neighborhood? If so, there might be a way to make the HOA take care of the issue as it is part of everyone's sewer and not just yours.
I would get it scoped out. Most plumbers have camera systems that can go 100's of feet usually and it is pretty easy. Then you can see where the issue is. If there is a weird bend, maybe that can be dug up and fixed.
There could also be a slope issue. Sometimes a sewage line will sag over time due to settling of the surrounding soil. It could cause a low spot where things slow down and then it all compounds until there is a clog.