RI won't clear up after several treatments, any advice?

[Bogertophis]

The thing that jumps out at me is that she's in a wooden vivarium: if the wood is not perfectly sealed, wood can absorb & re-share all sorts of bacteria+ even when
kept visibly clean. That's one reason I like glass tanks for housing snakes...it's the best to clean -plastic gets tiny scratches in when wiped, even from paper towels, &
over time may increasingly trap debris. I do realize that wood is a great insulator & that where you live, you probably need that, but what I'd do is change to a glass
tank & insulate that...sometimes you can even find a furniture cabinet to modify & contain your tank (you can install it on a slide-out too, for example) or custom-build.

If it's too cold now where you are to take your snake out for some 10-15 minutes of real sun (at both safe & adequate temperatures), it wouldn't hurt to try the UV, IMO.
I've seen snake's appetites perk up & subtle but apparent health benefits from such doses of natural sun, yes even for nocturnal snakes. You need all the help you can
get, dealing with resistant pathogens. Is this your only snake (reptile)? If not, I hope it's quarantined away from all others.

What also might help is some nutrition to keep her going & help her immune system. It's understandable that she doesn't want to eat right now, & I do not condone
"assist (force) feeding", but if she was my snake, I'd do a tube-feeding (liquid nutrition) as it's much more gentle for the snake and doesn't teach her to fear her prey.
If you'd like instructions & more information about this, I'll be happy to PM you my write-up. It's not something you do as frequently as the snake might eat either...
but it's helped many snakes recover that I've dealt with over many years of keeping many kinds. It is not that hard to do, I promise, & you might also share it with
your RSPCA Reptile Rescue folks.

And