Just throwing out some ideas. I had the same problem with a non responsive vet, too busy to spend time with a horse that needed extensive emergency medical care and more
interested in focusing on easier quick turn around calls. (long story) The suggestion to get a new vet usually isn't too helpful in most areas there is ONE large livestock vet.. Anyway,
I would be inclined to think they got into some kind of acidic poison based on the lip ulcer. Perhaps the donkeys were more inclined to sample a plant or non food item than your other
livestock. There was a big
temperature drop and freezing rain, is it possible you put out some kind of ice dissolving commercial salt for roads?? If he is still eating I would keep feeding him something easy on the
stomach lining like alfalfa and give him time without doing anything other than basic nursing care. Roll him, raise him up in the sling keep him clean and well hydrated. I bet he'll be ok
in about ten days. I was watching my neighbor's cow herd one day and noticed one particular cow was eating sassafras leaves which I thought was odd. Since then I've noticed she regularly
samples plants like privet and rose of sharon that the other's ignore. The donkeys may just have wilder taste in foods, just like some people.
I know he's just upset...I really think he thought we'd be able to pull him thru...guess I should of called other vets....I just didn't want to hear another euthanise him theory when he was still eating....IDK guess it is what it is, and we do the best we can. 
