IME having to stop multiple times on a climb and not feeling well is not a training volume/intensity issue. What you get in that case is mostly excruciating pain in the back/quads/hams/calves/triceps/ etc., which may or may not lead to cramping on the climb. In those cases, I've always been able to just keep riding, understanding that I was moving my perception of a pain threshold to a new level. That feeling of just can't turn the cranks is an energy problem, i.e. low blood sugar from some cause. The usual cause is what I call "sloshy stomach," meaning the feeling that one's stomach has contents, but they don't move. So my guess is some variation on that. What one does in that case - the case that you've feel you've eaten plenty but can't ride - is keep drinking water until you feel better, sometimes lots of water. You have to get the stomach osmolality down. So being a little dehydrated can be how this might start. Conventional wisdom among randonneurs is: whenever you feel down, discouraged, having trouble keeping going - EAT! (and drink).
On a serious ride like this, at least serious for you, eat 3 hours before the ride, then nothing. During the ride, try to eat no more than ~50 calories every 15'. Start eating like that from the start. Never stop eating for more than 1/2 hour. You were probably eating enough, but maybe too much bolus and not enough dribble. Once you start with the gels, you have to keep them coming. They're quick energy and quick gone, which is fine, but you have to realize that, and realize that gels take a lot of water per gel, say 6 big swallows.
Edit: as others have said, this is not a bonk. It's just low BS.
Re hydration: how often did you pee? My metric is pee every 3 hours. If I don't, I'm dehydrated and I'll sit and drink until I do. The late ride cramping is probably from overdoing it a bit. That's normal. The dry skin is not normal - you were dehydrated. When you feel that, it's late in the game and you're not far from becoming a medical emergency. Sit and drink plenty of water ASAP along with electrolytes.
During a ride like this, you should feel a little thirsty all the time. If you don't feel like drinking, you aren't using enough electrolytes. OTOH if you are really thirsty and are thus drinking and peeing too often, you are using too many electrolytes. The balance is quite tricky and thus I always separate my water and electrolytes.
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