James,
Yes you can check in early. You might use the SEARCH function on this topic and TheRustyScupper (who is a Fort CastMember) has posted some great info on the subject (I give him a hard time and try to keep his ego in check, but seriously he is a good guy).
Bottom line is you can check in as early as 630am. Every arrival is pre-assigned a campsite. Sometimes that site will have someone checking out the same day you are checking in. When they leave a Fort CM needs an hour or two to blow/clean/prep the site so it isn't immediately available. If you arrive before the official 1pm time, you can check in early IF A SPOT IS AVAILABLE. This is the little "wing it" section. If you booked a Premium and your Premium pre-assigned site isn't ready yet, you can ask to take possession of another Premium that is immediately available. But...
It may not be in the loop you wanted.
It may not be near a Comfort Station that you wanted.
It may not be near a Bust Stop that you wanted.
The best scenario is to check in early on a weekday during the non-busy period and be flexible.
EARLY = getting in at 630am is practically a free day to me
WEEKDAY = less arrivals than the Friday or Saturday weekend/week long stays
NON-BUSY = low times of the year when sites usually are available at all types
FLEXIBLE = if you are hung up on a specific single loop, it might not be available. But the one next to it might be.
If your pre-assigned site was vacant last night, CONGRATS. You will get it right away.
I've requested specific loops and arrived at 11am to check in and REALLY wanted that loop (1500 which is a Tent Loop) and I was told I was preassigned a loop in 1500 but someone was checking out that morning and it had to be prepped/cleaned. I said I would wait for it since I wanted 1500. The CM will not tell you your pre-assigned number but they will generally tell you the loop (the fine art of negotiation - always be polite and pleasant with the CM checking you in). The CM took my cellphone number, we went off-property to lunch, and we got a text during the meal saying site 1521 was ready for us to just drive down to and set up.
I'll be staying at Lake Louisa State Park in Clermont the night before I arrive. I'll be getting in around 6pm, do grocery shopping at the Clermont
WalMart to stock the fridge and chill the food, and check in at LLSP for $25 after hours. I'll stay hitched, just plug in, then leave in the morning for the 30 minute drive to the Fort. LLSP is a state park, not an RV resort, so set expectations accordingly. But I've done it before and it worked well. Once I get to the Fort I hate having to leave for groceries or paying for a full day (night) and only spending a few hours for that rate.
At least that's my plan this week, anyway.
Bama Ed
PS - this is basically the same policy as WDW's hotels. If you get there and a room of the type you've reserved is ready, you can check in early.