New pen and pad for the never ending needs and wants list!
Sheets, pillows, blankets and towels since you will be staying.
Know your height and check your route. A truckers atlas or RV/Truckers GPS are good tools. Google maps will not tell you heights.
Water regulator and Watts are the best IMHO, so if your starter kit has one use that to get by then invest during the offseason. You will get much better flow in your rig vs the preset cheapo ones. I made the change this year and it was significant on the flow rate.
An adapter for 50A to 30A Dogbone then possibly to 15A (for home) if the dealer is not providing.
Make sure the furnace and water heater is working before you depart.
If your unit is winterized and you are going to use the plumbing have them connect a hose to confirm no leaks. Some dealers bypass this in PDI this time of year as they don't want to re winterize.
Have a sewer hose, multiple connections for the hose to the ground pipe. Usually a Rhino kit has this, but if you have 2 connections you might need more hoses and connections. If this is well equipped dealer showfloor and they are offering a percentage off of toys take them up on it and while your rig is there match it all up. That stuff is no more expensive doing it that way vs piecing together later. You could get by with one hose, but you might as well get everything you need as you will anyway.
Waterhose is another thing that comes to mind.
Last and I feel most important is to make sure that you understand how to correctly de-couple and re-couple the rig. Being new to 5'ers and probably not too many fellow campers around this time of year to ask. Take notice that your hitch is fully engaged. Do a brake check tug (fully engage the brake controller) with the legs only 1" off the ground once you reconnect. It can save you thousands in a new bed if your trailer is not fully connected.
I have seen it happen and just had a new to RV'ing friend total his bed as result of being lazy and only listening for the "click". Just because you hear it does not always mean it's fully engaged and each hitch is different.
Other than that enjoy and congrats!
Sorry so long...