Covering combines

Tarps are not very good.  They flap around in the wind and tear, and they can also trap moisture under them.  Keep in mind, they are NOT completely water proof either.  I am not a big fan of them if you can tell.
The best thing is to find a structure to park your combine in.  It will deteriorate rapidly when left outside year round in the elements.

I agree that a shed or lean-to would be best. I use some heavy duty tarps to cover some of my implements, but I always make sure to leave them up from the ground to get air to blow up and circulate. I have had good luck buying from "tarps direct" on-line. I get the 8 oz super heavy duty brown tarps. I looked up a 20x20 that sells for a little over $70 plus shipping. They are heavy enough they don't seem to shred up from the wind. 

Park in a level place & face it to north.
Open both clean grain & return elevator panels.
After cleaning out unload swivel, place a 4"x4" wood block under unloading auger at it's transport saddle to prop end of auger upward slightly.
Remove inspection panel at unload auger swivel.
Vacuum out any excess grain &for dirt inside machine.
Place peanut butter bated mouse traps in cab & check, replenish bate everyday for 4-6 weeks.
Remove engine batteries & hook them onto trickle charger somewhere warm.

I covered a WD45 with a canvas for several years during the winter.  I put a plastic coffee can on the muffler to avoid the pipe tearing thru.  I tied it from side to side thru the grommets like a shoelace under the engine.  I put cinder blocks on the tires and the front bolster.

I don't know where you can get canvas any more, or how big you would need, but that is what I would be looking for if you are real concerned and don't want to be out in the weather replacing a tarp every couple months!

remove the raddle cover, drop the header, clean out center of auger and feeder beater, open elevator boots, open trap door, blow and clean till there is no more grain left.

Perfectly clean they do not deteriorate very much.