Al wire on old trailer home, 120v on breaker but 50v on hot wire to outlet.

There is a loose connection, probably at the back of a receptacle. Aluminum wire needs special attention for any connection. May be best to check them all, and may need to convert what ever connection is there to one better suited for connecting Al wire.

There are copper wire pigtails with a special connector for connecting to an aluminum wire.

Well I guess you have a voltmeter. Now try to explain your issue a bit better. What outlet, what wire? If taken simply, you have a bad wire. You need much more description for anyone to help... maybe you have a bad voltmeter... or a bad neutral... or a ground that is completing the circuit... and not a very good ground. Help us to help you.