This summer we were traveling in the mountain west and mainly used the hot spots on our AT&T iPhone 5s. I also have a Verizon MiFi 3 g card but didn't activate it this summer. In most places the 4g on the iPhones was fast enough for our needs. Especially if the service was 4g LTE as it was about as fast as our home service which runs on a fiber optic line. We get/buy a block of 10 gigs, shared by all our devices, for a month with any overage at $15 a gig or part thereof. In June we got our usage up to 9.85 gigs but never went over. My wife watched a couple of movies using her iPhone to hotspot for her IPad and it took about a gig and a half to watch a movie. We have 5 iPhones and a mini IPad that are usable as hot spots for the other 4 iPads the family has that are wifi only.
The worst service we found all summer was in Colorado Springs at the Garden of the Gods campground. Cell service was so weak we couldn't make voice calls but could send text and emails as the data service was better. Yes you can use the hot spot on the phone and talk on it at the same time with most all the "smart" phones available I believe. We could go a few blocks east or west of the campground and have good cell service. Something was blocking it at our location, not too surprising in the mountains.
I too have about given up on trying to use campground WiFi set ups. First I try the phone hot spot and use it if we have a decent signal, if not then I will try the park signal on their wifi. I prefer having the hot spot on my phone over a mifi card as I always have my phone with me. Helps when all the adults in the group have their own hot spot to use, so if my wife or I are away from the camper, the other person can still surf the web or whatever they want to do over the air.