A volt meter that plugs into an outlet inside the trailer. It'll let you know if the power is sufficient coming into the unit. Bad power supply can = dead A/C and others. Put it in a spot that is very noticeable, so you can't miss seeing it.
We just use hand signals and the occasional yell for backing...no walkie talkies needed for us. Stinky slinky for draining the grey and black tanks. White hose - get a good one to begin with and you won't have to replace any time soon. Sounds like you've got about all you'll need except for food and a bottle of wine for the maiden voyage would be nice - keep the cork for a memory thing.
