Strange to me that they should be that difficult in texas. Perhaps the car saw service in the north for part of it's lifetime.

Anyways when difficult use a torch set to heat the connection up. This is what I have to do quite often up here in the northern rust belt. Many times one of fittings has rusted down to a substandard size and vice grips become required.After the heating. The heating has burn off the ozidized layer of rust on a nut. Making it an odd size.

Penatrating oil of a decent type is the other possibility as mentioned if you can allow or have the time for it to work.