I haven't seen too many other suggestions for the Northeast yet. Here are a few of mine, that I think might have some appeal for younger children, a list that is by no means exhaustive:
The White Mountains of New Hampshire—the Mt Washington Cog railway, Franconia Notch (several neat things, such as the Flume), the Kancamangus Highway (and a number of the little stops along the way, e.g. Sabbaday Falls), and Clark's Trading Post. There are many, many hikes in the area ranging from short and easy to quite grueling.
Boston—there's a good many neat things to see in the city, but avoid driving in the city. Driving under the city isn't too bad, rush hour excepted of course, but propane is not permitted in the tunnels and even if you don't have the RV you still have to emerge somewhere and park. A number of the outer subway stations have parking lots or garages available, and that or commuter rail is how I would get into the city. Anyhow, the Children's Museum is good, the Museum of Science excellent (including a very impressive Van de Gaff generator show), and there are some great historic sites such as Old Ironsides, among other attractions. The New England Aquarium is also superb.
I like Canobie Lake Park, a traditional family-run amusement park in Salem, NH.
In Vermont, the Shelburne Museum is huge and eclectic and quite fun. The Fairbanks Museum (in St. Johnsbury) is a moderately-sized mostly natural history museum that seemingly hasn't changed a whole lot in maybe a century. It has a neat vibe to it and a few rather unusual exhibits; many kids like the insect mosaic pictures (assembled from preserved bugs). Mt. Ascutney is a fun (and rather popular) mountain to go up, partly as there's a road to get most of the way to the top. They have observation tower and several outcroppings with good views within a reasonable walking distance. The state park also has a campground (at the base); like all Vermont state parks it has no hookups available.
Although probably a bit out of the way, Watkins Glen (the gorge itself), between Syracuse and Corning, NY, is quite lovely and popular with families. It's not too far of a drive from there to Knoebels in Elysburg, PA, which is another absolutely fabulous traditional amusement park—probably my absolute favorite. They have a campground on-site. (The campground isn't exceptional in itself, but being a couple minutes' walk from your RV to the roller coaster is incredibly convenient and helpful.) Knoebels is free entry, with either pay-per-ride tickets or an all day pay-one-price wristband option, so it's quite possible to have a great time without spending one's life savings.