Pam has really said my thoughts regarding maybe feeding more...my previous boy was 15 too but as he was on steriod he did keep his weight up, but any change in diet would result in either tummy upset of excess toileting which proved stressful to an older dog.
If you give more treats etc or it results in more toileting - now my Harry who is 18months was given pure smooth peanut butter as he was really skinny pup, he still has it (guess its half tps ) on bit of my dry toast or slice carrot just in morning, but my late boy would not of tolerated it.
If she is well and happy think maybe best to leave well alone and but it down toher age., as with my old dog it was sad that we had to really monitor is weight gain in later life, how he loved a wee bit of sausage