Short answer: Check out Technomadia.com
Longish Answer: Most campgrounds - especially if they are in a rural area can only subscribe to a "Homeowners" Internet Package - or at best maybe a "Small Business" package. If it is just you and 2-5 other RVs in the park, the internet should be great. Problem is, when other folks move in and start using bandwidth it will bog down proportional to usage (and the Camp Hatteras CG I know of has 400 sites!
A good analogy here is imagine if the campground hooked up the camp water supply to their garden hose. GREAT water pressure when it is you and 2 others. ZIP water pressure when 20 more folks move in
Now to the next issue: WiFi Signal Booster. I have one of these (WiFi Ranger) and swear by it. It is great for a local VPN and I have Persistent WiFi in our Coach and around our campsite. It is great for pulling in weak signals - it will NOT, however, make a overloaded system work better.
Since you mentioned your own cell carrier as WiFi, 2 suggestions: take a look at a Cell Signal Booster (I recommend the WeBoost 4GX RV) and a trip to your cell provider for a plan change.
As of earlier this month, every major cell carrier (Verizon/AT&T/t-mobile/and...Sprint(gag!) offer "Unlimited" plans at a price point that should be reasonable for most folks (@$70/month) There are catches, but most plans offer you 22GB of data before you are "Network managed" (NOT Throttled) and 10-ish GB of tethered data (i.e. use your cell phone as a hot spot).
For more information I do invite you to check out technomadia.com - these folks make a living evaluating internet plans and hardware while fulltime RVing