Double check your manual? The Seiki 50" I'm finding on Amazon all list Clear QAM support, along with NTSC and ATSC.
If you're not getting channels after a scan (you do have to scan for each new park), the cable is using encryption, and you'll need a box from the cable company.
could be the park is just providing the feed, but you need to sign up with the 3rd party cable company to actually receive service (trailer park up the road from me does this)
quote from Amazon for the Seiki SE50UY04:
This TV contains ... over-the-air ATSC/NTSC RF input support, Clear-QAM digital cable support, digital/analog channel auto tuning, V-CHIP/CCD/Favorite Channel support, and more. I