Gaining weight??

Mercedes has always been a skinny dog and sometimes she's worse then others. She refuses food when I try and give her more. She likes just eating her small meal of 2 cups 3x daily. I uped it to 4 cups for supper lastnight and breakfast today and she accepted. YAY! wont take her long to notice tho and she'll refuse again. sooooo my question is what can I give her to help her gain healthy weight?? The vet said to let her free feed and this is not possible. Oh and shes a great dane about 4yrs old. Is there anything in the RAW diet some of you guys give that will fatten her up??I bought soem IAMS gravy and am puttign that on her food too. Have been giving her healthy table scaps once in a while.... Any ideas?

Does your vet feel she needs to gain weight?

Ruffles is on a high calorie canned food, I have to buy it from the vet. That has helped her to gain weight.

The vet said she should gain some weight yes, but she wasn't overly concerned. Mercedes has been up and down her whole life tho so its not an all of a sudden problem. I will check with our vet to see if they have the high calorie canned food. Thanks!