Haven't performed this task myself, but I've helped another guy replace his.

Not a major job by any means BUT it is VERY IMPORTANT to make sure the waste tank is completely empty. Tipping the bus away from the outlet is a great idea..... and still be prepared for a little "spillage" which is unavoidable.

It's also a good idea to make sure the replacement valve is well lubricated when installed, as it will ensure smooth operation for a long time.

Good luck!