At The Home Depot, I bought several gallons of Behr Multi Surface-Cleaner & Mildew Stain Remover. It wasn't expensive, $3 something for a gallon. OK, it did a decent job on the Trek Deck when used with a power washer. When I was done, I had about a pint left. I was all set to discard it when I noticed on the label it can be used in the bathroom. I thought, "What the _____?"**, I may as well try it. I put it in a spray bottle, sprayed the tile/grout in the shower/tub area, and it did a great job. My god, a gallon would last a lifetime, it you're using it in the bathroom. Oh, I took the shower curtain and held it to garage door with magnets. And it, with very little brushing, removed the mildew from it. When using it in the bathroom, ventilate, it does give off fumes.
** You fill in the blank, the missing word could be 'heck' or something else ending in 'ck'.