Hi there, I see that you requested a new answer and after reading about your dog's situation I have some thoughts I'd like to share if I may. While it is true that a high HCT and HGB can indicate dehydration, another thing that can causes these to be increased is a condition called polycythemia (many cells). It also helps to know the degree of the increase of these numbers because as with most things in medicine, the art is in the interpretation of the lab results.
When the blood work was done was your dog clinically dehydrated and did she have a concurrent condition that would be expected to result in clinical dehydration? Was she having any clinical signs or was this routine blood work?
Was a urinalysis performed? If so do you have the results (specific gravity of the urine can tell us if she is conserving water or not and help determine her hydration status when taken into account with other factors).
Was a chemistry panel performed with this blood work? If so were there any abnormalities? Dehydration is often accompanied by an increase in the BUN as well as an increase in the total proteins.
Was her physical exam unremarkable? Does she have a heart murmur or any other pre-existing condition?
In the absence of physical exam findings consistent with dehydration, a reason that she would be dehydrated (excess losses above ad beyond that which she can make up for such as vomiting, diarrhea or heat exhaustion) or other lab values that support dehydration or evidence of excess losses it is hard to make a case for dehydration as a cause of the increase HCT and HGB.
Assuming that she has a normal thirst response, if she were dehydrated she would want to drink water and lots of it. Since she is not wanting to drink a lot, the 2 possibilities remain that she either lacks a thirst response (which is la very uncommon situation) or she is not dehydrated and the increased HGB or HCT is caused by something other than dehydration. Have you considered consulting with a veterinary specialist?