Campgrounds and for that matter hotels put in WiFi that is not designed for great distances and with a business office there will be repeaters to spread the signal around but not in these other places.
Each repeater/bridge in a campground would need to be attached to AC power and be weather proof and theft proof so it is not a simple installation.
We have greatly improved our signal reception and connection by using a business class Wifi router with 3 external antennas (on the back of the router). Our newest laptops also have better signal reception as they appear to have better internal antennas in the case that houses the LCD screen.
We use the free Wifi whenever possible so we do not eat into our Millenicom GB data cap.
In the USA we have to be creative as internet access is very spotty outside of major cities in many states, including California where I live. We not use a cellular phone plan for data but instead a separate data plan through www.millenicom.com that is month to month and uses the Verizon cellular network and its towers which provide the best overall geographic coverage in the US at this time. As we cannot even get DSL at our house we use the Millenicom service at home as well as on the road in the USA.
We do see time of day issues as many of the places where we have stayed have only a DSL connection to the internet and it is being shared by everyone in the office and in all the campsites.
The sites nearest the office at a private RV park are going to have the best signal strength and it can help to know the location of their device as it may be at one end of the building and that side will have better signal strength than campsites on the opposite side.