I know little about your specific coach except that the 40DFT does have only one hydraulic slide, the rest are gear motor activated.
I would assume from looking at the manual (https://www.monacocoach.com/resource...007_Knight.pdf
) page 99 that there is a single hydraulic pump with two hydraulic lines, one to each end of the slide. When positive pressure is applied, the slide goes out, and when negative pressure is applied, the slide comes in.
For whatever reason, the hydraulic cylinder on one end of your slide is unable to retract the slide. This could be because of a cylinder malfunction or it could be because the slide has an obstruction or some damage that is causing it to jam. Once jammed, of course, the problem just gets worse if you continue to attempt to retract.
If you are able to get to the cylinders in your basement it may be of value to unbolt and remove the cylinders and manually push the slide in with a bunch of people (strong ones if you can find them) to manually push the slide in. I've seen this done, it is quite possible if there is no obstruction or damage to the slide. Once it is in, the slide can be secured with straps from the the cylinder mounts to something stationary in the basement.
If this doesn't work, there is probably some damage to the slide causing it to jam, or an obstruction that is causing your problem.
You can diagnose the cylinders by removing one end of the cylinder from the slide (without removing the hose) and running them in and out. If they both run in and out without the slide attached, than I think you are back to obstruction or damage.