I agree, that is the first place to look. Think of it this way: PWM from a VFD output if a modulation of frequency, a.k.a. FM radio transmission. If the output conductors are free to radiate through unshielded cable or non-ferrous conduit, they make a very effective transmitter. I see this problem a lot in water installations because people use PVC conduit to avoid corrosion problems, unaware of the fact that on a VFD output you must then use shielded cable inside the conduit.
A TDS meter is essentially a very very sensitive conductivity meter. To be that sensitive it will pick up all kinds of stray EMI, from an RF source or otherwise. I've had guys call me up on their walkie-talkies to tell me that their TDS meter is malfunctioning, then when I get there it's OK. I point out to them that it only malfunctions when they key up their mike! I've started to see that with come cell phones now too.
If that wiring checks out OK, then the next likely source is it being emitted from the line side. Most VFD manufacturers sell EMI filters that are connected to the line side of the drive for that very reason. Just because you don't read harmonics doesn't mean it isn't emitting RFI.
