Most Desired Accessories

Not mentioned so far: WiFi range extender, if you're traveling with grandchildren look into a washer/dryer, breaker bar with socket for emergency removal of tire, tools suitable for tightening things and quick repairs, consider a portable waste water tank.
For us, the very first thing out the door was the lousy Denver mattress. We put in a 12" bed in a box, which helped a lot.
I recommend that you provision your kitchen with new pots/pans/towels, flatware, cooking utensils and the like. I keep crystal stemware and bar glasses...a conceit, but I do not like to drink out of Styrofoam (it's hard on the flavor of good scotch, or bad scotch for that matter).

A Surge Guard (A good one is over 200 MSRP) is actually cheap insurance I like hard wired for assorted reasons including security and the "Oh it is just one night"

Hughes Autoformer can be helpful in many parks

Water filters are handy too. I made my own el-cheapo and it worked rather well. I used either a Brita or PUR filter pitcher for drinking water.

You will likely like a better (and longer) sewer hose and perhaps water (fresh potable) as well.

Extension cord matching RIG's needs (30 or 50) and adapters to everything Rig to all other sizes including 15/20 amp (standard house outlet).

Three light outlet tester
Voltmeter
Box of assorted fuses

You may find the need to lock cabinets and drawers Assorted methods ask if you find the need and I'll describe some

If you like campfires
Axe, Sledge Wedges both flat edged and pointed (That pointed one was amazing)

Outdoor lighting.

Thanks everyone. My MH is coming with water filtration system, WIFI range extender and stacked washer/dryer. I will be taking my complement of tools, hardware, outdoor lights, shovel, axe, sledge, telescoping/folding ladder, tarps, etc. Also plan to purchase an HDTV antenna with tripod. Decided to pass on the satellite and we have Roku with Hulu, Netflix and all the free stuff.

Has anyone found a good security system or the need for one?

The unit probably has a TV antenna already built-in; it may well be all you need, rather than having an external antenna. Any television antenna is capable of receiving OTA HD programming, of course, since it's on the same frequency bands as traditional television programming; there's no such thing as an HD specific antenna (except to marketing departments).

I did forget water filters Valterra adjustable is the lowest I'd consider

I used both a Watts and when it wore out a zurin (Since I could not find a rebuild kit) both were about 50-60 bucks with hose adapters. both were 3/4" (turns out 1/2 inch is more expensive???) went from static (set to 50 psi) to shower at full flow (Still 50 PSI) and the pressure gauge on the output looked glued in place (it was not) The elcheapo jobs that gauge dropped like a rock in freefall.

The three things you need the most
Master Card
Visa
Checkbook

Security systems: Good: Duke the Boxer dog
Better: Mom with her .38 S&W
Best: Me and Mr. Glock
Honorable mention: Nosy neighbors with good intentions

LOL!