Your vet is absolutely correct. Mild dehydration can cause elevations of RBC, HCT and HGB. The blood work elevations are very mild. When your vet performed the dentistry he/she probably gave IV or sub-Q fluids to normalize your dog's blood work. Older dogs may drink a bit less because the hypothalamus in his brain, which controls thirst doesn't work as well as in a younger dog. Try to get your dog to drink more water, especially in the summer season by placing some ice cubes in the water, or putting down separate bowls of dilute pedialyte, Gatorade, or even dilute vegetable, chicken, or other broths. Some dog's prefer running water the best, so purchase a doggie or kitty water fountain. They are not too expensive and worth it if it keeps your dog hydrated.
Thank you so much for answering my post. I feel so much better now!!! Thanks for the ideas, I'm sure they'll work.