Well, without seeing her I can't tell you if she's underweight or not, nut I will tell you I disagree with your vet on how you can tell if a dog is at a good weight or not. You SHOULD be able to feel the ribs BUT not see them, does that make sense? Now, will Yorkies, you won't see ribs through all the hair, but I'll tell you, my Akita is the perfect weight, per more than one vet and based on body condition scoring, and I can feel her ribs and spine, but they're not protruding out by anymeans. Alice is a thin little girl, you can easily feel her ribs and spine, but my vet explained to me that she's not necessarily "under weight", just on the low side of normal, not unhealthy at all though (it's actually easier on their joints to be trim).
As far as the food, I wouldn't use Royal Canin, it's not good food since it's full of corn and other fillers. I completely know where you're coming from as far as the whole sniff and walk away thing lol. I've tried quite a few different brands of food with Alice and she really seems to love the Nature's Variety that we're using now and going to stick with. Something "grain-free" tends to be more appealing to them, so look for something like that. Either NV, Orijen or another holistic brand. Before I found that she likes the Nature's Variety (kibble canned and raw) I did find that she really liked Evenger's canned food and would eat the Orijen kibble if I mixed it with that. You can also try mixing smoe shredded plain boiled chicken breast in with a good high quality canned food (there's many to choose from).
Good luck - post a full body picture if you can
