We have the 20MRB, your trailer's smaller and younger brother. We love the Murphy bed. Nice to have the floor space during the day.
As for your questions, have the dealer show you how the slide works.All the way in and out. When in, are the seals even on both walls inside? Outside, while the slide is in, is it in the same on all four corners? It should be tight.
You will probably hate the bathroom faucet, easily replaced with a higher spouted one, and I would suggest swapping the shower head for something better with a shut off at the head. While you are in the trailer, try and sit at the dinette. Can you get your feet in and out easily around the legs & feet of the table? (We replaced it with a card table).
Turn on all of the appliances on both propane and electric and make sure they work. If you can, place all jack stands down to stabilize the trailer and see how much it rocks & rolls. We bought these two items to eliminate much of that:
X-Tended Fit X-Chock - BAL 28014 - Chocks & Levelers - Camping World
BAL Lock-Arm Stabilizing Bar - BAL 23216 - Stabilizing Jacks - Camping World
I also bought some long under bed type clear plastic storage type totes for the outside storage bins. Keeps things organized (hoses, extension cord, tv cable, etc).
A foam topper helped on the Murphy bed and it will still retract all the way with the foam and bedding!
Hope I don't sound negative. Like I said, we love our 20MRB and have made the above mods to make it better for us.
Congrats on your new rig!