You don't need to do anything with the underbelly.
Look under the trailer. See those hoses hanging down? One drains the fresh water tank, the others are your low point drains. Unscrew the caps and let the water drain out.
Your water heater is behind one of the vents on the side of your trailer. It's a smallish box looking thing, with a plug on the side. Remove that plug and let the water drain.
Your water heater bypass valve can be hard to find. Mine is in the bathroom, behind what looks like a cabinet. It's a simple gate valve - turn it so it's closed.
Now, screw the drain caps back on underneath the trailer and start an air compressor. 30psi will do the job just fine. One by one, open your faucets and let them go until all of the water has been forced out. Don't forget your toilet. Then open the drains again and pour pink antifreeze down all of your sinks, into the shower and down the toilet.
All of this took longer to type than it is to do. It takes me about 15 minutes, start to finish.