We bought our 35UN a month ago and took maiden voyage last weekend, Memorial Day.
Yes, there is a screen on the emergency window, they will install it when the unit is prepped to go.
Since we have only camped on shore power I can't say about master bedroom ventilation. We always have a fan running in there anyway for white noise and use A/C for cooling.
Your A/C only works on shore power or generator power (120 V AC) and will not run off the batteries.
First mod you will do is install a latch on the sliding counter top that flies open on every right turn! All my other drawers and things stay put OK. I aired my tires up to 95psi and got better handling too. I also adjusted the mirror mounts outside on the brackets to get better views from the side cameras. There is a wrench for this in your owner's packet.
If you like how your rig looks now - wait until you wipe it down a couple times. There was a lot of "factory overspray stuff" on mine and as it came off the unit looked better and better - very shiny.
If this is your first tankless water heater, set it at lowest setting (coldest) then turn on shower hot only and see how it is. That was all we needed and even then, it was a little too warm. We were using water that came into the RV at over 70 degrees though so if you have a colder supply, you may need to turn it up higher.
We were very pleased with our first trip and all the important stuff worked as it was designed to.
Be sure to sanitize your fresh tank and lines before use. Then I recommend you spend a night in the driveway so you can see what you forgot to pack. I bought a soft water conditioner and put it inline with the supply and that was a great investment. Soft hair and skin and soap that bubbles!! Easy to connect and operate with fantastic results.
I ran a sub panel from my house to the area where I park my RV so I can plug in and keep it powered up while at home. Easy to do if you know electricity or have a friend who does. Depending on the length of run, it only cost a couple hundred bucks to do this but well worth the effort. You can find wiring diagrams online for this. Your RV becomes a guest house then!
Good Luck with your new Jayco and may it keep you safe everywhere you travel.
2015 Jayco Precept 35UN
Towing JEEP TJ with Roadmaster plates and bar w/Blue Ox Patriot brake
an Aluma low bed trailer with HD Road King and/or Polaris RZR onboard