New Food (again)

OK, I've talked about Gypsy and her food eating habits in the past. She is the least food driven dog I've ever seen. In puppy class she used to spit out treats when I gave them to her.
You can't bribe me with stupid treats!!!
So much for positive reinforcement...

On top of not being food driven, she looks like a starving refugee when she has short hair. I think the most she has ever weighed is 40 lbs. When she was younger the doc always said she was in great shape and it was better than being overweight. Now the vet is asking us to try to get a few lbs on her.
When it comes to dog food, we've tried it all. High end, low end, home made, raw, you name it. She usually will seem enthusiastic about a new food at first but then cool off and just nibble. Some days she might only eat 1 - 2 cups at most.
We switched to Merrick puppy food a few weeks ago. Our thinking is that maybe the extra calories would help pack on the pounds. She seems to like it and has not turned her nose up to it yet. We mix the wet puppy food in with the dry sometimes.
I'll take her to the vet for a weigh in after a few weeks.