Here it what I did for my windows
. I do not have a bath room window as My 13 footer doesn't have a bathroom but the kitchen window appears to be the same.
step 1. clean around the window.
2. blue tape around the window leaving about 1/16 inch gap between the tape and the window frame.
3. remove the window.
4. clean the window frame & fiberglass trailer where the window mounts.
5. put the window back in its opening without any sealant on it.
6. note the areas that don't touch the fiberglass when you put the window in place. These areas have to be built up with extra butyl putty. One of my windows
needed 2 extra layers in the corners on top of the single layer all around the window frame.
4. On my Scamp
the inside trim ring is installed backwards because of the thin wall structure.
5. remove the window and fit the trim ring backwards as used on thin walls. the trim ring must be able to go all the way down the window frame. When I received my used Scamp
the small frosted window trim ring was split. All of my other trim rings were not split. All of my windows
are radius windows not jalousie windows
6. After you verified that your trim rings are fitting properly and right way around mark them so you can put them back together properly.
7. with the frames on the window check the gap between the trim ring and the window frame. once the screws are tightened all the way the ring cant go further down. this is the gap you have to fill. Hopefully this gap is narrower than the thickness of your wall. I had no issue with this gap on my trailer as I have the thickness of the fiberglass shell, double thick reflextic and rat fur liner a at minimum 1 layer of butyl tape. In your case you probably have a shower liner too. If this is not thick enough for your trim ring you will need to add some more Butyl strips to take up the gap. I don't know if you also add a layer of butyl under the inside trim ring to seal the shower side.
8. install the window with the necessary layers of butyl with extra as needed in the areas that don't fit flat.
9. tighten the screws working around the trim ring a little at a time until the butyl starts to squeeze out all around the window frame. it is important the screws don't bottom out before the trim ring clamps up and if the trim ring bottoms against the window don't go cranking down on the screw it wont tighten any more and just breaks the screw or strips it. I replaced all of my old screws when I resealed my windows. Old screws are easy to strip the heads on giving you lots of trouble later.
10. after you you get the window installed let it sit a day or so that way the butyl releases its tension. then take a dull knife and trim the butyl around the window frame. Now pull off your tape and reveal a nice clean install with out a bunch of goop to clean up. When trimming the Butyl best to do it cold first thing in the morning. Once the stuff warms up it can cut into a gooey mess.
11. some people run an extra bead of sealer around the window to cover the exposed butyl. I don't but some people do.
12 Use real butyl tape. do not use putty tape. test what you are using. stretch it like you would a piece of chewed bubble gum. it should stretch smooth and slick for over 6 inches or more when warm. putty tape will only stretch a couple of inches if your lucky and where it breaks it will look grainy.