I've been using the same Barker 32 gallon blue tote since 2003 and use it for both black and gray and never had a problem. I replaced the wheels a couple years ago. I literally ran them into the ground.
Word of advise. Get the biggest one you can possibly afford. If your tote is bigger than your holding tank you'll never overfill the tote. The first time you misjudge and overfill the black, plus the contents in the hose, you'll regret ever getting a small one!
About black and gray. I've NEVER been anywhere that I could not dump black and gray at the same place. About cross-contamination, I've never heard of a place that is reserved exclusively for Gray water (but somewhere it's possible something like that may exist).
Cleaning the tank is no problem either. Dump your black first, rinse it out at the dump station, go back to your camper and then dump the gray. At the dump station, rinse again, and that's it! If you're real anal about your tanks, squirt some dishwashing liquid in the tote, add a gallon of water, and swish it around, and then dump. This will clean and it will have no smells, although if you always dump your gray last, you won't have smells either.
In the fall of the year when camping season is over, I take a few minutes to wash it out one more time at home. I use a little bleach water, slosh it around and just dump in the yard. (I live in the country). I rinse and then put about a quart of pink stuff in it to keep the valve (since mine has the side valve on it) lubricated over the winter. Sometimes I'll just apply a liberal amount of Vasalene on the slide.
Remember, bigger is better! And get the one with the slide valve, not the drop down hose. Those hose systems are nasty and messy. My in-laws have one like that it's awful to use. It makes a horrid mess.