Having been there, but not with quite your same dilemma, a similar thought occurred to me as Steve's after seeing the last added photo - adding pipe to get the flexpipe fitting actually out into the sidecase. Yesterday I was going to suggest removing that extra counter casement, but wasn't sure how much that might entail; I had to remove a piece from ours similarly, but it wasn't quite as "attached" as yours. At least now you have significantly better access!
The solution (so far) in our case was a longer flex pipe. You could add an extension of solid pipe, either to the black top pipe with attached cleanout or to the white PVC screw-on adapter, so it reached out into the adjoining case, then attach a PVC elbow turned to open toward streetside (assuming that's a curbside kitchen). You may or may not need a new flexpipe in order to get the length right, but use every mm of available space between the top and bottom black fittings up to the front of that case to allow the longest possible loop in the flexpipe. That means that when the slide is in, the curve zenith in the flexpipe will be right up against what I assume is a removeable case cover like ours - the one facing toward the streetside.
This is likely going to take some micromanagement like it did with ours, so without actually being there I can't anticipate what might need to be done that's much different from our solution. It may take the application of Roger's or Greg's ideas as well. Since all those other hoses and such travel as well, and the flexpipe may no longer be strapped to them or vice versa, probably they will take some modified management with the wire ties.
Now, I was tempted by angling the flexpipe's mount to the bottom fitting, upward 15 degrees or so (ours cracked at the bottom fitting); but double-checking that notion in practice prior to cementing things together, proved it put unexpected strain on the hose at the fitting much like what caused the original problem. The stress at full slide out is different from the stress at full slide in, of course. You'd have to sort that out for yourself given the situation is somewhat different, but I ultimately just stuck with straight horizontal flexpipe mounting.
I'm suggesting this idea since you need to get rid of that side-to-side stress on the flexpipe. By taking the hose entirely into the case and alleviating the turn to one side at the top, and freeing the hose from the restrictions of being wire-tied to the other tubing bundles, the flexpipe may be less susceptible to stress at the fittings. My one concern there would be keeping the now freed flexpipe from hanging up on or being restricted by the other bundle(s) of tubing and such in that narrow, confined case. The loop needs to move freely in either direction. Even using another type of hose, as some have done, won't dispel that issue. I stayed with flexpipe because of its slick and smooth inner surface that gray water gunk won't adhere to.
All this being said, it's also possible that just replacing the flexpipe with a longer hose and removing the restrictive uppermost wire ties from its configuration may allow the loop, now free to flex more in the case, to absorb more of the side-to-side strain, so the mounting point is less likely to fold or tear. Perhaps a rubber coupling as others have tried, fastened securely with hose clamps and placed near or at the top mount, would suffice to help distribute strain better - as long as the rubber is flexible enough, doesn't serve as a gunk collector inside, and doesn't split over time. If so, no solid pipe extension nor elbow would be needed.
A big thank you to Carol for adding that last photo!
Joel