Canned food has quite a bit of water
in it. You mention that she is on a special prescription
diet, but not if it is dry or canned. If it is canned, that might be part of the reason why she is not drinking much, but if the subQ fluid soaked in really fast, then that does sound like she is dehydrated.
Some dogs will lick popsicles. Some dogs will drink pedialyte or gatorade. Most dogs will drink canned, diluted, beef or chicken broth or a weak broth made from bouillon cubes.