My priorities for accessories would be:
1) A good high quality surge protector
2) .....
Have a Sewer Slinky - use it sometimes - especially when camping for longer periods. I really don't leave my grey water open most of the time - preferring to have a good bit of water in the tank for flushing the black tank. I assume you know that you do not want to leave the black tank valve open. Many parks even prohibit that.
Backup-cameras are very nice - but you have to remember that you can't see everything and still need a spotter.
You can get some of those parking pads, lego type levelers - or a 2x8 piece of treated lumber and cut it into several short sections and four sections which will fit under the rear tires to level the trailer. I'm sure you will be leveling it a lot.
You are ahead of most of us if the DW is willing to learn to drive the rig, how to park it, etc.