Grindstone is a fair distance off I-81 (about 25 miles and 40 minutes or so), with a couple relatively steep, curvy, but short grades to pull on Rt. 16 between the interstate and Troutdale. Nothing that should cause you any concern though, just something to be aware of since you asked. Rt. 16 is a wide two-lane "highway"-type road.
Rt. 603, which takes you from Rt. 16 in Troutdale to the campground, is narrower than 16 and curvy in places (more of a "country road") but is paved, and is no problem if you take your time. It basically follows the creek valley down, so no real grades to speak of. Lots of camper and horse trailer traffic on this road, never saw anyone having any problems.
Grindstone campground sits in the bottom of the same valley right at the foot of the mountains, and overall is relatively flat for this part of the country. Some sites do have a little pitch to them, but you should be able to find multiple suitable spots with no problem most times. I am pretty sure you can reserve site-specific in Grindstone, so take some time to look at the pictures and maps on the Reserve America site, which allows you to search for sites by length, etc. and will show you a photo of the sites. Most of the time you could "walk-in" and get a good site, but the 4th of July weekend will usually fill up. I would definitely recommend reserving a site that weekend.
Grindstone is a great campground, well maintained and lots of shade. Since you will be in the area a while, think about checking out the wild ponies at Grayson Highlands State Park, although the route from Grindstone to Grayson Highlands would be a little more of a challenge than the road in, but still definitely doable.
Good luck, safe travels,
Regan
