Now on to serious stuff:
A surge protector for the electrical cord. While it may never be needed and may not work like it should when it is needed, the little test button lets you trip the power so you can disconnect trailer without going to the post to kill the breaker. Remove load or power before disconnect.
As Pro said, get a 50 to 30 amp adapter. Also get a 30 to 120, a trailer 30 twist lock to a 120, and .........well, there is probably another that one too.
Get a funnel, hose reducer, water jug with spout....some way to get water into the fresh water tank. Also get a second length of hose. It's a pain to have to move the trailer a few feet when the 20ft hose is too short. It's also a pain to haul out a longer hose when it is not needed 9 out of 10 times.
Get a clear sewer section. Really helps to see that the flush has worked. Really liked that cam lock system, going to have to check that out. There are some interesting solutions to handling the black water hose. Worth researching a bit. The nitrile gloves are a bit better quality and likely more sanitary than rubber gloves unless you wash the rubber frequently. Get an use a separate hose for black water flush operation.
Get a dedicated battery water container. Distilled water comes in gallon jugs and you will never need that much. A pint or quart bottle will work. Some type of dedicated funnel is a help as well.
Get a clear coat touch up kit. Your baby will get a scratch.
Get inside and outside mats, step covers, a short handle stiff brush to clean them and big garbage bags to pack them when you can't clean them first.
Get a small grease gun and refill cartridges. If your hitch does not need to be greased, the ball does. One of those reusable grocery bags (not the paper) can be used as a cover when not towing. A big garbage bag is a good idea when you load the hitch head in the trailer or TV.
And that is just a start. The folks who have been doing this for years have lots of great solutions. Keep asking.
Now who has a solution for formed dividers that fit the curves of the overhead compartments? Certainly, that is a need that someone has filled!