I'm new to this myself, having just brought home our Escape in the last month (used not new), but there are a few things I would add to this, all gleaned from others posts on here and the fiberglassRV forum.
-An adapter for plugging in at your house
(needed unless you have it wired with an RV outlet).
-An adapter for the RV parks that only have 50amp service
(what the bigger RVs use).
-A heavy duty extension cord
in case the site is badly configured.
-A clear elbow for dumping
, so you can see when you're done/if the solids are actually coming through.
-An elbow for dumping
that fits various dump site configurations.
-A pack of fridge bars
. Don't store anything in the door while traveling.
-A water filter for drinking water
. Attaches to the hose you already have, replace once a year.
-A water bandit
so you can fill your fresh tank from the pumps in state parks.
-A toilet brush
, because I gather the toilet gets dirty quite easily without a lot of water flushing debris away.
-A dish drainer
and leveling blocks
. Often a legal requirement for towing a trailer, which depends on the state. I believe WA technically requires it, but it's also just a good idea.
Also a lot of people recommend a second 25' drinking water hose in case the site is badly configured, and a separate hose for cleaning out the black tank. Personally I got the second hose for drinking water, but didn't bother with a black tank hose.
Hope this helps, and hopefully some old timers have some better ideas than mine!