I usually like to open the toilet valve and reach in as far as I can and give the tank a good scrub with a toilet brush, warm water and liquid detergent. I then fill and drain, repeat, repeat, repeat and then give them one final scrub. I then fill with nice hot steamy water and fabric softener and then drive 100 miles and drain. I do this after every trip. Same process on the grey tank minus the brush. When I’m done they look like new tanks and smell wonderful. My friends and family can’t believe how great my tanks look.