How long can dogs live if they are still eating, but drinking and urinating very little?

My GSP is 13, has thyroid cancer and debilitating arthritis in her remaining back hip and knee. She is still eating every day, but is not drinking much, and has yet to urinate today (it's 11:30 am). This is the first dog I have ever seen through an entire natural life span, and I would like to put her to sleep, but my husband does not, so we are doing palliative care for her pain. She can't stand up, and I think she is having difficulty voiding her bladder because she doesn't have the strength to squat, though she doesn't appear to be in any distress. She also has not done a #2 in several days, perhaps for the same reason, or perhaps because we added Tramadol and Gabapentin to her pain meds. Any ideas how I can get her to drink more water, and help her urinate? Any ideas on how much longer she may live (I know you're not psychic, I am asking days? weeks?)