If you really HAVE to go to WEM and get late leaving, try the William A Switzer provincial park about 20km north of Hinton off the Yellowhead highway, the Gregg Lake CG there accepts reservations and there are hook-up's.
There are 3 NP CG's near Jasper that accept reservations and have hook-up's Whistlers (the largest) Wapiti and Wabasso (the smallest) and 16km south of Jasper
Do beware that in July these CG's fill up quickly and I would recommend reservations if you can.
I agree with the railway comments about Lake Louise, if you want to camp on the Bow Valley Parkway between Lake Louise and Banff, Castle Mountain is probably the quietest since it's the furthest from the railway track which runs parallel to the Parkway between these two places.
As an alternative stay at Kicking Horse CG in Yoho NP only about 25km west of Lake Louise on Hwy.1, no hook-up's here but there are showers you MUST get here early as it is very popular and no reservations. If you stay here be sure to visit Emerald Lake and Takakkaw Falls.
Trevor