It is possible you might get lucky and get one campsite, but for your group, unlikely to get that many in Jasper. I would suggest staying on in Banff if you are already in a non-reservable campsite, which means you have it as long as you stay there. If you are in a reservable one, you will have to move on. If you must move on there is an overflow campsite that you could move to between Castle Junction and Lake Louise you could use to get through the weekend. If you do move on, I would also suggest finding a campsite along the Banff-Jasper highway, and spend a few days. The best bet is Waterfowl Lakes, about 35 miles north of Lake Louise, as it is all non-reservable, and it has a dump station as well. Be aware that some of the smaller campsites along the highway are older, so have narrow roads, smaller sites, and may be difficult for large rigs. Good Luck.