I think the inconsistency indicates a problem with the incoming signal. I know with our Continu.us TV, the picture quality can be quite good if the source is HD. My OTA digital channels look GREAT. Most RV resort-type places we have stayed that have cable have it in SD only (just hook up coax from their port on the post to the port on the outside of my trailer, turn the antenna booster off and do a channel search on the TV using the "cable" setting), so it looks like crap for most of the channels (a few of them are HD, but like OP said, those aren't usually the ones DW wants to watch). Of course, it's cheaper and easier for the park to do it this way, and they can put "Cable TV" on their amenities list.
I've only stayed at one park that actually sent me to my spot with an actual cable box that I hooked up in my trailer. I used the HDMI ports, and the picture looked great. Well, as great as that little TV can do. But even then, the signal coming into my trailer was through a COAX line on their post.
All that being said, it's probably a good idea to give your connections a good once-over. Make sure they're all clean and tight. But even the best connections can't fix a crappy incoming signal.
2014 Jay Flight 28 BHBE
2015 RAM 2500 6.4L HEMI, Tradesman 4x4, 3.73
Blue Ox SwayPro (BXW 1503)
Upgraded from an REI internal frame backpack and a Eureka 1/2 dome tent!