You've probably figured out something by now, but you may need some sort of line extension or put the spare tank on a chair or something so that the propane line off the camper can reach it.
I number my camper's propane tanks, 1 and 2. That corresponds with odd and even years - this year I was using tank #1, as 2017 is an odd year. Now, at the end of the year, I take the used tank off and use it for the grill on our deck at home over the winter. In the Spring I fill it up, put it back on the camper, and switch over to the other tank and start all over again. This way I always start out in the Spring with full tanks.
With weekend camping, we have never even run out of one tank. As others have mentioned, you may just want to check that the switchover valve is working correctly and there are no leaks or anything else not working correctly.