We use a 2HP 6 Gal. compressor for winterizing, adding air to tires, rotating tires with a impact wrench, blowing out dirt and dust from many things, etc. Well worth the investment. As for winterizing this how I do it.
1. Drain fresh water tank, close valve.
2. Drain hot water heater replace drain plug.
3. Dump and (flush if possible) both black and gray water holding tanks, leave gray water valve open.
4. Screw compressed air adaptor into the fresh water inlet. The adapter is available from Camping World or most RV dealers.
5. Apply compressed air, keeping the pressure at 30 PSI but less than 40 PSI. You may need someone to hold the air hose on the adapter.
6. Open then close each faucet, hot and cold, one valve at a time, allowing the compressed air to force the water out of the line. Don't forget the shower and toilet and outside shower. Open both low point drains and close.
7. Remove the drain plug from the hot water tank and allow the compressed air to blow out the remaining water. Reinstall drain plug.
8. Remove the compressed air source and adapter. Close gray tank valve.
9. Pour a few cups of RV antifreeze down each drain and toilet.
10. You’re done