You want to replace the anode rod after you drain the water. As far as the actual functioning of everything, it won't make any difference. But here's why you should put the rod back in place.
1) to keep small bugs out of the water heater. You'd be amazed at what will attempt to crawl inside that holde.
2) to keep the threads of the water heater and the threads of the anode rod from rusting up and making it almost impossible to put the rod back in place later. It only takes a day and the bare metal threads will begin to rust. Even the threads of the water heater hole will rust, making it almost impossible to re-insert the rod properly. Keeping it in place will prevent the rust and also make it easily removable the next time.
In my opinion, the best way to leave the water heater is "empty" when not in use. Draining it after ever use accomplishes a few things.
1) it drains the particles out from the eroding anode rod. All those little white flakes of stuff will spew out the hole, keeping all that from building up in the tank. Then you take off and road motion stirrs all that up and then you turn on your faucet and all those white particles will clog at the sink faucet air-screen at the end of the faucet.
2) if draining, you always replace with fresh water. It's amazing how conscious folks are on these forums about keeping their lines sanitized and all that, but then will let water sit in the water heater for a long time without draining. Drain it, it will be fresh water next time you fill it up.
3) along this same thought here, if you drain it, there's no water in the tank to grow bacteria.
A petcock?
No! You do not want to do this. Modifying the anode rod will defeat the purpose of the rod. The rod is there to protect the water heater. It's designed to be threaded and in contact with the body of the tank. It's designed to deteriorate as a sacrificial element to protect the tank. Modifying it in any way will only cause damage (in time) to your tank.
New Rod?
You do not need to replace the rod until it is completely deteriorated. If you drain your water after each use, you will find your anode rod will last much longer than if keeping water in the tank. I owned my Springdale for 8 years. I think I replaced the anode rod once, because I drained after every trip and never stored the camper (even a week) with water in it.
I've had my Outback for 21 months now, and I pull the rod and drain after every trip (still), even if going out every week-end and parked at home for 5 days. I've noticed there is very little wear on the rod so far, after 21 months of use doing it this way.
For all these reasons, drain the tank and put the rod back in until the next camp out.
FYI, I drain my fresh water tank completely also after EVERY trip.
