I pull a 24 ft enclosed for the race team I work on. We have cabinets up front, with a full work bench. I have a winch under the cabinets so if we wreck the car we can pull it back up. I have a mini fridge as well as AC. I have overhead lights so we can work on the cars all night long to get ready for a race in the morning.
Atleast for me our biggest things is tools. Tools tools tools and spare parts. I have bins of nuts/bolts/zip ties and so on.
I have another cabinet of medical stuff, paper towels, oil...
For you, obviously a generator is nice. Honda makes a nice quiet one. A mini fridge is so nice to have, as is camp chairs hidden somewhere.
You could easy put a water tank in, but you must be care where you put all that weight so it doesn't make your trailer sway when you pull. The position of the car in the trailer has alot to do with the ride back.
It takes about 30 min to get an Evo 8 in and strapped down correctly and lined up where I want it.