What you are planning on doing is exactly what I do. I leave my trailer up in the mountains from Thanksgiving through April - skiing every weekend. When I arrive, if there is significant snow on the slide out I remove it, but typically I just leave it. I leave my slides out all winter.
When you leave, winterize with a small air compressor after every use. Drain all tanks then hook up the compressor to the city water inlet. Open each faucet (hot and cold sides) one at a time and let it run until only air comes out. Open the low point drain and leave it open so that collecting water can run out during the week. Unhook your fresh water hose and alllow all of the water to run out. Pour some RV antifreeze into each if your P-traps.
That is really it. The whole pack-up/winterize process takes me about 35 minutes. Never had a major problem over the last 7 years.
Other things:
* Buy a good dehumidifier and run it constantly while you are up.
* Buy an oscillating ceramic heater. This will significantly cut down on your propane usage. * Buy an AC powered hose heater so that your waterline doesn't freeze on really cold nights.
We love it!!