IIRC, if you are an American citizen you need a passport to re-enter the U.S. Good luck getting one if you don't have one. The wait time, even for expedited processing, is months.
Check on the status of the municipal campground in Sparwood, B.C. Stayed there in 2009. Decent place with free firewood. Make sure you stop at the world's largest truck ever built in the center of town.
The only thing I can think of in Eureka is the town campground right across from the Town Pump gas station/convenience store. (The campground further up on U.S. 93 no longer allows tent camping. Think I saw some power outlets there. Not isolated at all. Big grocery store a little farther up U.S. 93. If you go off route on MT 37 there is a large, federal campground in Rexford. You can reserve some of the sites. Nice bathrooms with flush toilets and power outlets. No showers, but the restaurant/bar in town sells showers. $6 in 2019. It also sells some of the best fried chicken and Jo Jos on the planet. Seriously. You can walk down to the lake from the camping area.
The Towne Campsite in Waterton Gillage is in a dramatic setting with great views of the mountains and lake. Worth a day off there to recharge before the tough day back into the states. Expect a headwind on U.S. 89 to St. Mary. St. Mary campground inside the park as hiker/biker sites, but if you can make it farther up Going to the Sun to Rising Sun Campground you have a shorter climb the next day. Only 2,000' IIRC. Broing plenty of nutirtion for the ride over Logan Pass.
On the other side of Logan Pass, I prefer Sprague Creek Campground. There is a large-ish infield area with tables and some tent pads for hikers/bikers. Smaller campground so not as chaotic and noisy as Apgar. Also, you can walk to Lake McDonald Lodge. Treat yourself to dinner at the lodge, but get there early. But first take a tour of the lodge and then grab a drink at the bar and take it down to thr lake. The camp store at the complex had mostly junk food.
