Unless your power cord is unusually short, you probably won't need an extension cord. I have a 50' marine shorepower cord left over from my sailboat that I use to plug in at home but I've never had to use my shorter RV extension cord at a campground. Your ideas about adapters and sewer hoses are good. Other things I'd suggest are:
1. Woven poly patio mat
2. Wheel chocks
3. Dually valve stem extensions if it doesn't already have them
4. A surge protector would be nice but probably too expensive as a give-away, maybe a nice discount on one (including install if it's not portable). Ditto on a tire pressure monitoring system (TPMS).
5. Freshwater hose, filter, pressure regulator
6. Emergency Warning Triangle Set - I'd recommend you buy this before heading out even if you can't get it for free (we used our's the first time out):
Good luck and let us know what you get.