Wow, Bezz. That's really interesting. I had no idea that they had made a different model like that. That sucks!
The best advice I can think of is to install a Portapotti or get a custom made fiberglass
box to mount the toilet on. I don't think a plastic box would be strong enough.
I'm pretty sure there isn't enough room under the trailer.
I would say if you're going to go to the trouble of customizing a tank, put it in the storage space under the bed. You'll be able to get a decent sized one in there. Of course you'll still have to raise the toilet to above the top of the tank.
Good luck my friend! Let us know how it works out.