
This is almost exactly the problem I have had with my 4 year old elkhound. she was much better after her litter of 9, but his lasted only for about 6 months. she isn't as bad now as in the first two years, but will have stretches of weeks where she may eat 3 or 4 times a week.
She is fit healthy and wiry, and everyone that meets her assumes she is the puppy of my 4! I found no matter what you did you could do nothing about it. I have just mated her for her second litter (her last babies were 2 in November), she had been eating very badly prior to her season and until the dog came to stay, now she is being a piglet, go figure! she started eating again once she had stopped standing for the male.
I console myself with the fact that she will be great to show in Veteran classes, as she will not have gone to pot, and will be youthfully fit. she has done reasonably well in the ring, but has always done this in spite of lack of body and coat!:D
As long as she is bright eyed and bushy tailed just feed her, if she doesn't eat, then offer it again at next meal, and stop beating yourself up about it :)