I don't know anything about the tank on your RV. If it is a "gaseous, vapor" like a typical BBQ tank, then you can just hook the two together with appropriate hose. THIS WILL NOT FILL YOUR BBQ TANK !
To do that, you need different pressures between the two tanks. The easiest way is to make one hotter than the other. Before you open any valves, put the BBQ tank in a large container of ice water. The more I've the better. Once the tank has cooled to below 40°F, then open the valve. Pouring hot water n the other tank helps.
You will have to keep this up for about 30 minutes. Yes, that is a lot of I've !