Reba,

I may not be of much help because I'm not a technician, and the forum is practically dead nowadays.  I am the only one that is probably checking into the forums, and that is primarily because I am the moderator for the forum.  But with that said, let me see if I can help.

I'm presuming that you have opened your grey tank into which the washer will drain.  If that tank happens to be full, it will back up and flood out in the places that may be the lowest in elevation in the RV.  That may include the shower drain and the washer drain.

If the grey tank has plenty of room, it may be that the washing machine drain is not getting enough air for the water to go down the drain.  That may require having a look at how much space is around the drain line from the washer and the size of the drain pipe.  If your first load worked fine, then I really don't know if this is an answer to your problem.

Sorry that I can't be of more help, but I am definitely NOT a plumber.

Terry