Dog peeing in house again

My 29 lb dog, Luna, has habitually urinated in the house since we adopted her about 5 years ago. She is about 7 now. She has always gone into my daughter’s playroom to pee. We did training with an in home trainer and followed her advice (basically frequent trips outside and lots of praise/treats). About a year later we worked with a dog behaviorist who suggested feeding her in the playroom, spreading treats around the room for her to find. None of it worked and Luna continued to pee in that room every few months. Over the years, she developed a pattern of doing well for a few months, then peeing two or three times, then going a few months before peeing again. We finally put a gate up to keep her out of the room. That reduced the problem, although she did pee in other rooms occasionally since the playroom was inaccessible. A year and a half ago, my other dog (male) passed away. After that, Luna went about 15 months without peeing in the house. We took down the gate and I thought the problem was solved. However, a few months ago she started peeing in the play room again. She peed 3 times this week! We use Natures Miracle and have a washable rug that we wash each time she pees. We have tested for UTIs multiple times and all bloodwork is normal. I walk her at least 4 times a day and make sure she actually pees when we are out. Any ideas on why she is doing this, or how to stop it?!?!