I agree with you. If this was a voltage loss type problem, you would be looking at codes such as P0740,P0753,P0758,P0787,P0973,P0976,P1860,P2761,P2764 and/or P2769 in addition to the P0785. There still is a chance that there could be a problem with the solenoid itself, the harness that runs to it, or even a powertrain control module type problem. Testing this problem at this point gets rather difficult and you need a scan tool that can command this solenoid on/off to see if the solenoid is getting the proper voltage and the proper ground to operate. If the solenoid is getting the proper items need to work, but yet is not working, you have a solenoid problem for sure. If the solenoid is missing the voltage side of things, we must really look at this as a possible problem with the internal transmission harness. If you are missing the ground side of this, we really need to first check the resistanct from the negative side of the solenoid to the powertrain control module. If this is found to be ok, you likely have an powertrain control module problem. Here is a wiring schematic of this system so that you can see this solenoid alone.
Feel free to contact me with any questions! Good luck!