And then get landlord insurance and a flood certification.I'd have a plumber scope the sewer line (blockages, cracks) and look for any nasty weak spots in the house in the plumbing (mold, water soaked walls, dry rot, termites, stains) just so you know what you are getting into.

With the next blockage you might have s strong indicator of the offender. 

As far as augering out your drainage system, you can rent a power auger and do it yourself (or hire someone) but be prepared for it to take several hours if the blockage is significant.

No damage, deformities, blockages or anything abnormal noted during the inspection.

Alternatively, there may be a situation is which the water is pulled out of the p-trap too quickly, siphoned out by inadequate venting or movement in the drainpipe, or some kind of blockage, or, or, or.Please don't tell me that not one of your four plumbers suggested this possibility.

Not sure what it is but I have a plumber going tomorrow  Probably a blockage in the main line.

This way you can seek specific help on blockages you have instead of just asking them to give it all to you.

No blockage.

The plumber can remove the trap under the kitchen sink or laundry tub and run a small snake through the pipe until he either clears the blockage, or hits it and can't get through.

I had to replace the entire cast iron sewer pipe system because of in-line blockages.