I'm going to put a few pics in here for reference in case they help. I replaced both dump valves AND the tank flush head while I had the tank pan off last fall. I'll admit it was pretty easy with the cover off....
That being said.... i'm not familiar with the set up on your specific Airstream. But, on my 2001 34' Limited the toilet is Street side and the flush head is mounted near the top of the tank, just above where the black valve mounts to the tank.
(Old flush assy. in this pic)
Other than all the sensor wires, there is considerable room above the black tank valve for maneuvering, again however, I had the cover off. One thing to note..
As seen in the pic, my factory installed flusher had a barbed end on it and was installed straight onto the pex tubing supply. The replacement I bought for repair had a female hose connection...
I ended up having to get a bit creative with some brass fittings.
Once that was figured out & I was ready for reinstallation, I generously cleaned the area where the fitting mounted to the tank and installed with a healthy amount of silicone and stainless replacement screws. DON'T over tighten the screws, just nice & snug.
I wish I had a better pic but in this one ( just for reference sake) if you look close up, you can see just the bottom of the tank flush flange, just behind the main exit pipe and above the old black valve.
A rough estimate of it's location, using the main outlet pipe(where it exits the tank cover) 20" towards the front & 7" towards the curb side.
The cover is made of galvanized metal and is about 1/8" thick. Cut as big of an access as your comfortable with and use similar material and the appropriate robots to "seal" up the hole when complete.
Let us know how it all goes...