The closest I've been to Caledonia was to drive by the entrance on US-30. Codorus I've been to twice, but not in recent times. You don't say when you plan to visit, but in season, Codorus is one of the few state parks that has a swimming pool and it's within walking distance of the campground. However, you might want to drive to the pool if you're in one of the loops that are further away, are handicapped, have small children or taking a lot of gear there.
Yes, Codorus is further than Caledonia, but it's a much larger park with more campsites and has a large lake. The closest town, Hanover, is the home of several snack food manufacturers and some (I've been to Utz's) offer tours of their plants- a fun trip for both adults and children.
A "water thief" will solve the problem of obtaining water from threadless water faucets. They're easily obtainable and every RV should carry one. The annoying part, though, is that with most faucets of that type, the shutoff valve is designed to close the valve unless someone holds it open.
If your "mission" is primarily to visit Gettysburg, You'd probably save time, money, plus have the convenience of full hookups booking a private campground in the Gettysburg area- and there are many to choose from. If your planned visit is "off season" I would think some campgrounds might even offer lower rates. I'd rather give my money to the campground rather than waste my time and fuel traveling between Gettysburg and a state park; especially if you're visiting for more than one day. 20 minutes? That's 40 minutes round trip. Now add another 20 minutes to dump your tanks on the way out (assuming that you don't have to wait in line) That's a whole hour you could have sitting outside your RV relaxing, sipping a glass of wine (or whatever your favorite beverage is).
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TV: 2009 GMC Sierra 2500HD | Crew Cab | Std. Box | 4WD | Duramax/Allison
RV: 2000 Jayco Eagle 266 | FBS | TT -- 1986 Coleman Laramie pop-up -- Still in the family!!!