I don't have the same trailer as you, but installed my washer and dryer myself.
Do a lot of careful measuring inside and outside as well. On mine I could measure from the window on both sides.
When you do drill, like you said a small hole first...... I had to move the center of the hole about 1/2" to have enough room to miss molding on outside of rv.
It's not as bad as it seems......
Then you can double check with the 4" hole saw before you use it to make sure all is good.
And like you, I do NOT trust dealers!!
Good Luck
