Shauna,
Sorry you had these problems. Don't think you have a lemon after your first trip but you certainly did not get good instruction on your rig.
-Sounds like you hepvo valve in the shower is plugged. Do not try to unplug it by running anything through it. This thread can help:
https://www.montanaowners.com/forums...ad.php?t=80569
-Remove your add on that connects the black flush with your fresh water. You don't want those connected in any way which might contaminate your fresh water.
-No clue what your washer issue is
-There is a troubleshooting guide for the furnace here on the forum.
https://www.montanaowners.com/forums/downloads.php Did you run it before buying the rig?
Hang in there it should get better.