It is difficult to get full understanding of the mounting from the pictures and it is probably just as hard to get it from being there and looking! Since we know it is highly unlikely that it was mounted from the inside of the van during construction, I would assume that it was unitized on that bottom plate and then attached. It looks to me from the pics like there is minimal distance inside the mounting box and it would be a booger to get out through the opening.
What would happen if you put a floor jack under the plate and then attempted to unmount the plate from the frame? I can't imagine that it is mounted to the plywood box shown to enclose it from the inside.
Also, I would rig up a small mirror on something and poke around inside there and see if I could see anything else that supported it. Like you, that looks pretty minimal on the attachment to the frame of the plate.
There aren't a lot of Horizons and Falcons out there and the odds of you finding someone that has removed the generator may not be too high.
I own a Phoenix Cruiser and once owned a Horizon, the little sister to the Falcon. They were built by Intervec. When Intervec went out of business, some of the employees formed Phoenix Cruiser. A guy named Kermit is president and it is a small family owned company. You might very well call Phoenix Cruiser, ask for Earl or Kermit and see if they could answer the question. Kermit is good to respond and very, very knowledgeable.
Here is the link: http://www.phoenixusarv.com/
Just trying to help a fellow Okie!!
Paul
PSW
2013 Phoenix Cruiser 2350
2014 Jeep Cherokee behind it
and a 2007 Roadtrek 210P for touring