Hi. Multiple myeloma (MM) is a very complex illness and kidney disease is associated as a secondary illness alongside it in about one-third of all dogs. As the MM starts in the bone marrow, there is good reason for keeping an eye on blood levels and - in particular, anaemia occurring due to low red blood cell formation. For this reason, it is actually fabulous that your dog is eating well, and I would never discourage her from eating as much as she wants whenever she wants. In terms of the MM alone, the overall response rate for dogs treated with melphalan and prednisone chemotherapy is 92%, with 43.2% of dogs achieving a complete response and 48.6% achieving a partial response. There is therefore every good reason to use this combination to treat the MM.
The survival time with treatment ranges, but the mean average is about a year and a half. This could be much the same for well managed kidney disease. So it may be tough working out which will claim her life eventually. In the meantime, I would just enjoy every day - and let her enjoy each day with you, whenever you can.
So, there is no problem with too much food - just let her eat the amount she wants to eat. What I would say is feeding one or two big meals is not advised, because that just means the kidneys have to work really hard to deal with all the toxic by-products produced. Much better to feed smaller meals more frequently, if that's a practical thing to do.
Secondly, almost all kidney failure dogs are dehydrated, because although they may seem to be urinating more, they are in fact not holding on to any hydration. They are evacuating water from cells, bladder, organs and blood - and therefore they NEED extra fluids just to stay hydrated. Let her drink whenever she wants to. As a precaution against the toxins that all faucet water contains (flouride, chloride and other chemicals that are bad for kidney disease) and high levels of sodium, I would suggest filling her water bowl with filtered bottled water, if that's a possibility.
Hope that answers your concerns.
Tony