Andym133 has a good point if you're still camping alot for the season. At any rate, if you're going to replace both- make sure you've thouroughly flushed both tanks and fully dumped before you start. Have gloves and a bucket ready, and some good disposable rags. one of the best tools is a small 4 or 5" crescent that opens to 7/16", or a very small ratchet with that same sized socket. If you want to lube the seals before you install the valves- get a small dab of white lithium grease and wipe it around the seal on both sides...avoid opening the gate valves because those seals will want to come out. This isn't a bad job at all and should take less than an hour.
Happy Camping.