There's about six or so hours of driving total between Bangor and Burlington, so you have plenty of time to explore.
Some things to consider doing include:
Ascending Mt Washington, either via the auto road (if your truck does not have dually wheels), the cog railway, or hiking if you're especially intrepid and pretty fit.
The White Moutnains in general have a lot to see and do. A detour to take the Kancamagus Highway (NH 112) is one nice option; it's entirely practical to pull the RV over this road, and even pull off in most of the pull-offs if they aren't too crowded. There are many overlooks, hikes ranging from a few minutes to a few miles, historic points, etc. along the road, and it's a very pretty drive.
St. Johnsbury, VT has a couple interesting attractions: the Fairbanks Museum is a primarily natural history museum but with some other very widely assorted items, and is full of character. In some ways, it's kind of a museum museum, showing how museums used to appear a century ago or so. The local library ("Athenaeum")_is also worth a stop as it has a very nice little art gallery. Both were funded by Mr. Fairbanks, of scale manufacturing fame.
There are a number of hiking trails and some State Park campgrounds in and around Groton State Forest. All the Vermont state park campgrounds have no hookups available--just a dump station and potable water fill.
While it's a little out of the way, crossing Smuggler's Notch (VT 108 between Stowe and Cambridge/VT 15) is a really neat drive. DO NOT attempt it with your fifth wheel. There are parts of the road that are very narrow and very twisty. Every year a few semi trucks manage to get their trailers wedged between the rocks, despite not being allowed on the road at all and despite there being significant fines for their taking it.
As far as campgrounds, the only one I have firsthand knowledge of staying at with hookups along the route is North Beach campground, operated by the City of Burlington. The beach is nice, the location is pretty convenient to Burlington, though the campground itself is unexceptional. If you have bicycles, it's right on the Burlington bike path. In the general Burlington area, one attraction well worth attention is the expansive Shelburne Museum with folk art and many rather random things (allow a full day). If you like baseball, and things are more normal next year and the team hasn't been eliminated by MLB/MiLB, catching a Vermont Lake Monster's game at Centennial Field is a whole lot of fun. The ballpark is one of the oldest minor league parks in use, arguably the oldest one depending on how you define things. It's inexpensive fun, if not always stellar baseball playing.
