Surge suppressor, either hardwired or portable. Water filter system. Screen tent (we use one with our 5'er, too). A 50ft. extension cord for your shore power, especially if you use state parks. A waste tote tank, again for state parks with no site sewers. 5-gallon water can.
A lot of the stuff depends on where you plan to use your trailer most of the time. Parks with full hookups or state parks with electric only...or out in the boonies with only what you can carry. And how long your typical stays might be.
It is easy to load up more stuff than you'll need, but after a while, you will start to winnow out the extraneous stuff and leave it home.