G'morning from Silver Springs, FL. Noticed your inquiry related to the Boundary Waters Canoe Area (BWCA) and as a former life long resident of Minnesota (Duluth) and a devoted BWCA user I just HAD to reply.
We usually haunted the area above and below the border (Canada) off the Gunflint Trail North of Grand Marais. LOVED that whole region. Now this was way before we ever even thought of our current full time RV lifestyle.
Gotta say I just don't think there are many, if any, bigger rig friendly camps along the Gunflint let alone in Northern Minn. as a whole. The municipal campground at Grand Marais is, as recommended by "teekay" above, likely your best shot. Don't know the exact reason, but Northern Minn. simply doesn't offer alot of campgrounds other than the parks and some private primarily for small rigs, tents, etc. I kinda think it's due to the very short season.
Gotta ask. Are you planning on doing some portaging into the backcountry with a canoe???? If so, I'd be happy to offer some suggestions and tips if you'd like. It's a wonderful experience.
Steve & Lynette
2014 Newmar 3103 BAYSTAR/Triton V10 w. Banks/05 Honda Element toad