I just winterized my 255BH which should not be too unlike your camper in method.
Very near your water heater (mine is in the very back), there should be two water lines, a hot and a cold, poking through the black plastic barrier. Those are your low-point drains and you'll need to unscrew both of those.
You'll also need to get to the water heater from inside the trailer and turn the valves to 'bypass' the heater.
Get a 1-1/8" socket and remove the sacraficial anode from the water tank (the large bung near the bottom front of the tank from the outside access port). This will drain the tank. You may want to flush the tank because there will be a bunch of zinc salt from the anode you'll want to blast out of there.
You can either pour antifreeze in your fresh tank and suck it through the pump or, if you have an air compressor, make a fitting to connect your compressor to the water supply port on the side of the trailer. The fitting costs about $8 to make out of 3 parts from any decent plumbing place or big-box hardware store.
If you do antifreeze, be sure to close the low point drains again. If you do the air compressor, you're in pretty good shape.
You also need to drain everything out of your fresh tank. You may want to pour some antifreeze into both your grey and black tanks and a little in the toilet so the seal doesn't dry out.
Does that get you started?
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