I'm right there with you. I never used the swirl o matic in mine. It had a crack in the plastic flange above the flapper - which the PO thought he could seal by smearing toilet flange wax into the crack.
I could have had that 3d printed, but when I pulled the toilet out, the aluminum cast metal had turned to dust.
Then, found out my fiberglass tank was cracked at the Thetford.
I've been building my own tanks out of black ABS. That allows me to install a new toilet with new waste connection where I want it. Also, I reconfigured the tank to allow me to put gray water in as well, and mix at the rear. You can find pics of some of this if you search my avatar.
As for the toilet install, I found the Dometic toilet at Vintage Trailer Supply. By chosing the short one, and the threaded toilet flange, you can dial in the exact height you need to reach the bench height. I then plan on cutting the back of the bench wall to get the toilet mechanics in there. There seems to be barely enough room to get the bowl in there. I had to disassemble the toilet to get it through the back access panel, and will have to reassemble the toilet once it's in the bench. I haven't cut the bench yet, but hope to have almost the same look as the original. I'll remake the bench cover just a little bit bigger, and hopefully you'll have to look twice to know it's not original. The flush valve will be a little tricky, so I need to work on that piece.
It's been a while since I started. We've had massive rain, and a lot of cold, so I don't have any pics of the toilet instal yet, since I haven't done it yet, but looking forward to actually have working gray and black tanks and toilet by spring.
My tanks I'm installing tank washers, and considering installing tank heaters too, but I'm not sold on those yet.