Router extender

For several years we used Bear Extenders they worked well, but only for one computer at a time, so we each had one for our laptops.

Now we have a WiFi Ranger which connects to the park, or any other open WiFI, and provides us with our own WiFi for all of our devices. Works really well and pulls in weaker signals since antenna is on the roof. At one point I compared and my laptop could see 4 WiFi signals, the Ranger saw 22!

I have one similar to http://www.alfa.com.tw/product_category.php?pc=3 but with two antennas that I got for about $69 at walmart. works great.
bumpy

Give this a try. I love mine.

This will pull in the weak signal, and allow you to create your own wifi network inside your camper.

There are a couple of options depending on how you want to do it. If you are just looking at putting a single computer on the WIFI, this is probably the cheapest and easiest option:
http://www.alfa.com.tw/product_category.php?pc=3
This will extend the coverage of the laptop WIFI, since it is external, you will also be able to move the antenna around to get better signal ie near a window.
If you have multiple devices (computers and phones) this option may work better for you
http://www.jefatech.com/product/RV-KIT-REPEATER
This is an internet router that will connect using a much higher gain antenna that would be mounted outside and the router would be inside. You would connect laptops etc to this router, and than it would relay your signal to the CG wifi. This has some advantages, longer range, ability to permanently mount the antenna on your TV antenna mast or Ladder, or use an extendable flag pole, you will also get more security as your devices will be secured behind your own router, which will help protect you from being hacked thru the CG open WIFI. This will require some installation/mounting, but is easy to use once installed.

Not trying to sell a brand and there are a number of other brand options that others may suggest, but these are really the 2 basic solutions that you can use.