That bolt head in the third photo is the sacrificial anode. You're right that removing it will drain the tank. But first, open the pressure relief valve to let off any pressure that might be in the system. The valve is the one in the second photo. My suggestion is to leave the anode out for a few days so that as much water as possible can evaporate, then reinstall it. I'd also suggest opening up the area under the port side dinette seat--you may need to remove a screw or two to get the lid under the seat off, and I just left the screws out once I'd taken them out. Once you can look into the area under that seat you'll see the piping that runs to and from the water heater. There should be a shutoff valve in both the red and blue pipes. Shut them both off. Then you'll see a short piece of pipe in the same area that bridges the red and blue pipes. That's the bypass line. Open the valve in it. Now you've bypassed the water heater, which you emptied earlier (make sure the water heater switch is kept shut off once the water heater is drained). When water runs through the blue line toward the water heater, it'll go instead through the bypass line and into the red pipe. That way you can still use the sink and shower faucets, you just won't have hot water. Later when you put antifreeze in the lines (if you intend to) you won't have to spend a fortune filling up the water heater with an antifreeze/water mix. And in the spring you won't have to figure out how to get all the antifreeze-tainted water out of the water heater.
A caution. When you take the lid off that dinette seat you'll expose the wires that supply both 110 and 12 volts, so take care. They should be partitioned off but you can still stick a hand in there. It's a good idea to make sure the camper is unplugged and the battery's negative wire is disconnected before inserting body parts in the area behind the electrical panel there. That's why the factory installed the screws in the cover I suppose.
In the floor under the sink are the two low-point drains. Pull up on these drain valves to open them--they snap open, they don't twist to open, then open the sink and shower faucets to let the water lines drain. Then you can do the air-pressure blowout to get almost all the water out. it's up to you whether you want to put antifreeze in the lines or not. I did, but I'm a belt-and-suspenders guy. Do put a cup or two of antifreeze in the sink drain. You don't want the residual water in there to freeze.
A few of us bought and installed water pump bypass kits last fall. That allowed us to put antifreeze in the water lines without having to fill the water tank with the water/antifreeze mix. We left the tank empty for the winter. That saves a bunch of money on antifreeze and saves the hassle of cleaning the stuff out of the tank in the spring. I winterized last year with just over a gallon of the stuff. You can search posts on this forum for a thread about the water pump bypass kits, I don't remember the thread title. Later--here 'tis: "A-frame hardside water pump question". it's from last October.
It's never too late to have a happy childhood!
Lee, WU0V, and Courtenay, N0ZDT
2011 Rockwood A128
2000 Silverado 1500 pickup
60W solar system
2000W inverter generator