Aggravating I know, but if a tech can't fix that, he / she needs to find another line of work. As dalengail said, very simple system. Motor, gear box, actuator, 2 gear packs / rollers, drive shaft between gear packs, then rails. That's it. Then of course the adjustments at the end of the rails.
If I didn't know any better, I'd swear some of the people you've dealt with used to work for Forest River. Sounds like some of them I've dealt with. Slide opening not even square, no problem, normal. Whole side wall leaning inward / out of square, no problem, just how they're built. Slide leaks, nothing we can do. I've heard all kinds of stupid crap.
John and Rebecca Dickson
Emma-8 / Little John-5 / Iva-1+
2013 Ford F-350 Lariat FX4, CC LB PSD, DRW
2015 Jayco Jay Flight 28BHBE (#8)