I can't speak to your setup but the Turbo 400 transmission has an electrical passing gear kickdown connection. GM installed a switch under the gas pedal which supplied the 12 volts to this connection when you floored it. You could slide under the rv, look for the wire and disconnect it to see if the transmission will now go into third. The downside of disconnecting the wire is the tranny will not downshift automatically.
Back in the day (Young and aggressive) I drove cars which had a modified th400 transmission which I removed the wave plates and accumulators, modified the valve body for instant shifts (bark). These engines had lumpy cams with little vacuum so the modulators were worthless. I had a plug for the modulator which simulated maximum vacuum. It would shift into third by 25mph. For go fast I would manually shift