David -
I think you may be making this more complicated than it has to be.
The Electra Magic's holding tank has a nominal capacity of 6 gallons. When it has 6 gallons in it, it is REALLY full, and I wouldn't try to move the TM in this situation.
The plumbing below the Electra Magic, between it and the outside valve, has a capacity of about a gallon. So if you fill the toilet to a full 6 gallons, and then pull the inside valve, the toilet will be left with about 5 gallons in it. In this situation, you can travel, but as you say, really gently and slowly. To avoid splash, some folks raise the seat and stretch plastic wrap over the bowl.
When you are ready to leave the campsite, it is OK to pull the valve on the toilet, then close the TM and drive to the dump station. Once there, attach the drain hose, and pull the valve under the trailer. This will completely empty the toilet and the plumbing below it. When it is drained, close the outside valve, remove the hose, and you can drive away. No need to open the TM at the dump station. Of course, you have not rinsed the toilet, so sometime in the next couple days, you should open the TM, close the valve on the toilet, rinse thoroughly, then dump again as above. When you are done, close both valves simply so their rubber seals don't dry out and malform.
Hope this helps. If I have missed something, ask again.
Bill
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