6pm is not too late. Once we arrived at big meadows around 8pm. Your reserved site will be available for you with your name and camping duration dates posted on your campsite.
Actually, no sites offer electric service. Generators may be used at most sites, but check regs on noise and hours.
If you miss check-in, find your spot or an empty spot, do it first thing next day. Only some sites have electric. Website is full of info.
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