In this reply I will address only fresh water as a single topic.
As for water hoses, I keep a 20 footer and 2-10 footers. That way I can connect at 10, 20, 30 or 40 foot lengths without a big snarl at the hook up point. If I need less that 40 feet I have an extra length for access to the spigot without disconnecting the service hose.
I have a water pressure regulator to protect the rig from the occassional over-pressurized Campground. I usually hook this up to the spigot directly unless the configuration prohibits it.
I have an in-line water filter at the input to the trailer to protect the inside micro-filter from larger particles. This allows my inside filter to last a bit longer and the macro-filter isn't expensive.
I have a Y-connector with individual shutoff valves to give me outside access to the spigot without having to disconnect the service hose from the rig. Has been very convenient on multiple occassions.
I have spare washers for hoses, and spare filter-washers as well.
I tend to spare in depth so that I can meet 90% of my requirements. Don't feel that you have to have everything I do, especially if your rig setup is marginal in weight.
A rubber or plastic sliplock wrench is also a handy tool for tightening and loosening water connections in case you have a minor leak, or have accidently over tightened a connection and can't get it apart.
That's topic one - a quicky. If I have time I will do AC power next.