I have custom ordered a $50000.00us living quarters horse trailer from the Hoosier manufacturer in Indiana. They want $20000.00 for a deposit. This seems ridiculous to me. What is the standard deposit to put down on a trailer?

There is no standard. If you are ordering something custom built they may have trouble finding a buyer if you renege. There also may be trouble with your credit rating that they may require a larger deposit. The thing is its up to the manufacturer. If they insist on $20K then you either pay it or take your business elsewhere.