You have to remove the portable tanks from the camper to get them refilled.
You can buy a temperature sensitive strip that glues to the lp tank and can help you determine how much fuel is in your tank. You attach the strip to the tank, pour hot water on the side of the tank where the strip is mounted, the strip will change color when you pour the hot water on it, the strip will change color less at the level of the liquid that is in the tank. Here is a link:
http://www.leevalley.com/us/garden/page.aspx?cat=2,40734,40996&p=55008