I have found that RTK (when properly calibrated) tends to point out implement issues but it sounds like in your cases implement issues are pointing out GPS issues!  It is a tough deal when expectations do not jibe with publish specs.  Just for the record I also have a guy planting with EZ-steer WAAS but I explained to him in great detail that nudging would absolutely be needed but he has his old markers to determine when that was needed.  That was fine with him as he just wanted to take some of the driving burden off his shoulders some of the time.  He is a happy customer.

On RTK systems I always try to have the customer there measuring the tractor calibration process so they understand the system is working.  Then when that is understood they better understand something else might be the issue when out in the field.  It could be anything but most likely not the system (if calibrated correctly).

Hang in there....the more you do it the better it gets.