If you have worked with her since she was 5 months old and she is tame, halter broke, and broke to lead she should be fine.
As far as washing her, I think she may have you trained. She knows if she acts up you will stop whatever you are doing. Tie her to a strong fence and put two gates or corral panels on each side of her. The first time, maybe just feed her there, and brush her. After a few days, try wetting her feet. Then slowly move up her body, with the nozzle on shower spray, not jet. Do not even attempt to wet her head until after she is comfortable with you wetting her body. After a few days, slowly wet her head. Again, she won't like it, but you can train her to put up with it.
Again, I only suggest the above if she is tame, halter broke, and broke to lead. If that hasn't been accomplished yet, don't waste your time even trying to wet her. Leave her home.
I will tell you, it's no fun being at a show with a crazy animal. It just causes you more stress than what's its worth. There are enough "stressers" at shows for a normal animal. At our recent show, we had kids on scooters, kids playing football in between the barns, etc. Although both activities are inappropriate in a show barn, it happens, and you've got to deal with it.
Best of luck!