Given her age and size, I would be inclined to think the stiffness is likely arthritis, and probably not related to her other symptoms.
I would put the skin problems to her age as well, thinking that as she gets older she has different requirements. Do you feed her fish oil? Fish oil is high in omega 3 which has anti-inflammatory properties, will improve the skin and coat, and studies show it also helps to prevent heart disease, cancer, and arthritis. A dog won't get enough omega 3 on a kibble diet because of the processing of the food. Same like humans who eat processed food they are high in omega 6 (which is associated with inflammatory response) and low in omega 3. Omega 6 and omega 3 should be balanced with each other much like calcium and phosphorus.
Personally, I would add fish oil (up to double the label suggestion for 3-4 weeks) and a good glucosamine/chondroitin/MSM supplement. Pick a good quality fish oil that is in a dark glass bottle, and preferably has vitamin e in it. Or use capsules. The pump-type plastic bottles you typically see are often rancid, or will turn rancid not long after you first use it. Capsules don't have to be refrigerated, but bottles of oil do (even if the label says you don't have to). Crappy fish oil is as good as none at all, and none is cheaper!
Maybe she does have the beginning stages of heart failure, in which case it would be imperative to add omega 3 to her diet to help prevent further damage, and help to try to repair some damage already done.