Hi,
At this point, this may or may not help you, but I'll share it anyway.
I know nothing about this motorhome, but our motorhome is about the same vintage and has dual electric slides with a single slide controller box. At the time, GBM did make a three slide floorplan and it had two (identical) slide controller boxes, as the controller could only do 2 slides per box.
If yours is wired similarly, it's possible the front two slides are on one control unit and the third slide is on a separate unit. Since your rear slide works, that controller is functional. IF it's the same as the front slide controller, you might try swapping controller boxes to see if you can retract the front slide. (By the way, we had slide controller issues on ours after the first few years of ownership. The original soldering was terrible, and while it was out, I replaced a the relays as well. For what it's worth, our slide controller was located on the floor behind the AC breaker panel and interior DC fuse panel.)
Even if the above is not helpful, there SHOULD be a manual retraction method outlined in the owner's manual. Or do a search online. You might have to release an electro-mechanical brake before manual operation can be done. Again, search around to see what you can find for that specific RV.
I feel for your father-in-law, my uncle went through something similar as old-age set in. I was able to keep him camping as long as possible, doing most maintenance and even driving the rig for him to the campground, but eventually he had to call it quits. I know that day will be in my future too. The age of 80 is only 17 years away for me!!! Hard to believe I'm that freakin' old! LOL
Good Luck,
~Rick
