Sandy, I went through the same thing you are going through with my girls. I worried myself sick that the girls stayed so thin. Chance was also very thin before his neuter, but that did the trick for him. Now he's the only one that has meat on his bones. Mine are all so short in length that there is a fine line for them. Not enough weight and they look lanky, as you mention, too much makes them look "frumpy."
I will say that after the girls turned a year old, more towards 18 months old that they filled out some. Not a lot, but more than they were. I think some are just meant to be thin. Their metabolism is higher than others. If you take a look at, say, Tracy's Brody, your little Tilly, Erin's Paige, JayR's Mimi, Bella Luna, Quinn's Phoebe, Kioana's late Honey, Lexie, Gia & Jade you will see what I'm talking about. They are different sizes, but built "thin." There are a few more built that way here, but some I'm not as familiar with. They don't have that extra layer of skin that some have. I've noticed that the thin ones all have what I call "knobby" chests. You can see their bones protrude in the front.
Anyway, I think I drove my Vet crazy worrying about the girls weights and being so thin.
He has assured me time and time again that as long as their ribs don't protrude (you can literally count them they will show so bad) and they aren't "sunk in" that they are fine. Another way that you can tell if they are too thin is if their head looks to big for their body.
I schedule feed all of mine twice a day with canned food. I also free feed kibble. You can try anything that is a "healthy" choice. Eggs, Chicken, Hamburger Meat (no fat), Cottage Cheese, Cheddar Cheese, Yogurt, Apples, Carrots, Green Beans, Peas, Potatoes, Peanut Butter, etc. You can also try some of the grain free foods. It comes in canned as well. Dogs don't need carbs to survive, but they won't hurt them. Some things may not agree with Tilly's tummy, but you can discontinue anything that seems to cause upset. I would go very slow introducing any of these things, though.
You can also get supplements that you can add to her food daily.
Try not to worry too much. Easier said than done, I know. I worried about this topic until mine were well over 1 year old. That's what Mom's do.
But I think she'll be fine.
This is about all the "meat" I can get on Lexie and she is 3 years old. My Vet said her weight at 3 lbs. is fine. To me she is a "sack of bones."
This is how thin Gia is at 2 years old. The Vet said her weight at 2.6 lbs. is fine. Since she has SO much hair, I don't "see" how boney she is unless she is wet.
Any recent pics of Ms. Tilly?