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The isolation transformer won't help.
Look at it this way: you want to differentially measure a small dc voltage that is not referenced to the chassis ground. A three pronged plug on your VTVM, one os hot, one is neutral and the round pin is case ground. The metal case is grounded via the third pin to protect against you getting a shock if the case is not at ground potential.
If you try to connect you IM-18 VTVM to the load resistor in the next step without removing the ground pin connection, you will short circuit the minus supply in the TV set and cause damage.
Background: in your chassis, to avoid adding a DC restorer circuit to clamp video black level, the designers coupled the DC component of the video from the detector to the CRT grid through two stages of video application through the 12AU7 amplifier. To DC couple the video stages, the video detector is at about -135v. The DC level on the plate of the first amplifier is coupled to the grid of the second amplifier. And the second amplifier plate DC is coupled to the CRT grid. The designers did not provide 100% DC coupling to avoid changing brightness as the 12AU7 warmed and current drifts. But the coupling is sufficient to give consistently good black level clamping.
You therefore must be aware that because of this design, you have to take precautions. RCA warns you a little bit. Their audience is trained electronic engineers and technicians and the warning would make them consider how to make the measurement under these circumstances. I hope this is clear and you understand. If not please ask more questions. If you do not understand the design concept of the video amplifier design concept in this set, it is a good time to learn.
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