Don't have a W/D in our unit, haven't installed one yet....not on the road enought to justify at this time. With that said, haven't seen where the exhaust vents are mounted on the exterior wall, height wise, and what type of RV vents are available. When Linda had her vent problem last year, cleaning the vent was a problem and should be taken into consideration as to type installed. With that said. I looked at the Spendide site and the only thing they said about the vent was to angle it down and away from the dryer. Which might place it about head high outside.
Like You I have the enclosed bath. Plumbing on the left and I have a rather large access panel on the bottom of the rear wall. Looked inside some time ago and there wasn't anything in it, so not sure what the purpose is. If you have the rear panel, might open it up to see where the wall studs are so you will miss them.
I haven't had to cut that size hole in the side wall. Have drilled a 2 1/8" and lots of 1 1/8" when working on the compartment doors. Assume the
HOLE SAW BIT you were referring to was one of these in the correct size.
The exterior wall is not very thick, PI's
CRAFTSMANSHIP page gives you an example and look. I had no trouble drilling into the fiberglass and shouldn't worry about cracking the siding as it is thick enough between the glass and backer to do fine. The one thing I would be aware of is possible kick back when starting the cut. Larger bits act differently than the smaller ones and shouldn't present a problem if aware of it.
Tape the siding beyound the cut area, mark your center hole and use the bit, pilot, to drill your center first. Should use caution when starting the 'hole' cut as it could have a little kick when it starts. I usually start off at a slight angle to start the cut that way not using the full 'teeth' to make initial bite. As the wall material isn't thick, use light pressure.
Hope something helped.
Chris