The road that goes past Buttermilk Falls is not paved, and the portion that leads to the falls from the south had YUUGE pot holes when I drive it last September. Even with my Forester I had to pick my way through.
Bushkill is on the PA side of the river, so riding there would require some time on U.S. 209. It has skinny shoulders in some places. Bring money if you plan to cross Dingmans Bridge. It's a private toll bridge and they charge for bikes.
I have ridden though the entire DWG numerous times, mostly on the NJ side. Not sure where your campground is (I have stayed at Worthington), but both Old Mine Rd. and N.P.S. 615 are nice rides. There is a steep climb between the road that leads to/from Dingmans Bridge (C.R. 560) and Peter's Valley Craft Village. (Old Mine turns into Walpack Rd. during the climb.) If you head north from the craft village you will be on C.R. 615. After a few miles you will come to Layton, where there is a nice, Greek-inspired restaurant/store. If you head south the village you will be on N.P.S. 615. N.P.S. 615 ends and you pick up Old Mine Rd. again there is a very steep climb. That climb is also usually very bumpy in place. On the left you will see a road towards Crater Lake. That's pretty, but there is more climbing. After the summit you descend down to Old Millbrook Village. Old Mine Rd. from there toward Worthington State Forest and I-80 is nice. Don't go up Millbrook Rd towards Blairstown unless you want pain. There is an incredibly steep section.
In the end, it's impossible to make a loop on paved roads using only the NJ side. The most benign riding in terms of hills is the northern Section of Old Mine Rd. between U.S. 206 and C.R. 560. Any climbing is very gentle, and you pass by the oldest house in Sussex County, NJ. Keep you eyes open for bears!