@gudde1251 and Jagiven
Thanks for your reply!
As for support, all I want to put under the tread is a wood board (riser) about 3/4" thick that runs the depth of the tread along the sides of the stairs (see rendering below). Those riser boards would basically hold the tread. I'm not sure exactly how I'm going to do it, but I could imagine to simply velcro the tread on the riser boards, for easy cleaning.
I also thought that a 3/4 inch thick tread would suffice as it only spans 26".
BTW the reason for my putting the RV into CAD is not the steps, they are simple enough, but the cat ladder in the rear. It was a headache to figure out and it will be a big help for me to cut the pieces now even while the RV is still in storage. Here's a progress shot of the cat ladder. (I hope the cats get it)
The cats can walk above our heads along the top trim of the slides. Here's a photo how it looks like while still under construction (no finish, no carpet clad) (see yellow arrows).
They can also use the cat ladder that I'll put up above the stairs as shown on the next rendering. I had a ramp there, but the ramp was spanning in front of the hallway window; I didn't like that.
(We got rid of our swivel chairs. The cats would have shredded them, and I hated the extra weight)
(Isn't it weird what people do to their RVs?)