My mother will not bathe or change her clothes. How do I get her to bathe and put on clean clothes without insulting her?

My mother lives with my husband and myself. She has her own room and her own bathroom full of shampoos and soaps and lotions as well as a ton of washrags. It has been a month since her last bath. Her hair is smelly and hasn't been washed in months literally. She says that she takes sponge baths but I know that she doesn't because the soap and shampoo are never used. I do the laundry and she never has any towels or rags for me to wash. Help. I don't want to hurt her feelings but something has got to give. She is totally mentally alert and walks with a walker. I got her in the tub once a month ago when she moved in with me and my husband. We love her dearly but can't put up with her not being clean. What do we do?