We returned from an all too brief trip to Calaveras Big Trees. Because we didn't have reservations and because the Jumping Frog Weekend was being held in Angels Camp and perhaps because there seemed to be a gathering of Harley bikers numbering in the hundreds around Angels Camp and Murphys, we could only get sites for two days, Thursday and Friday. $25 per night.
I believe that you are right to be suspicious of the 30-foot-RV limit. Quite a few of the RV sites would have been too short for our 25-footer. Then you add in space for the TV even across the front of the site and things get quite iffy.
Not all sites have water close enough to reach with a 25 foot hose and the bibs are not threaded. Worse still, some of the water faucets seem to be the handpump kind. However, if you need fresh water, the exit of the dump station has potable water with a threaded end. The dump station is just past the Visitor Center near the entrance to the North Grove Campground.
On Thursday we stayed at site 35, which is at the "extra vehicle" parking area and allows for quite a bit of maneuvering room, provided the area isn't filled with extra vehicles. Site 34 right next door is nice and level too. There are three street-side pullouts that seem suitable too. Numbers 21 and 30 have water and number 58 does not. 58 also has a somewhat confined entrance and exit guarded by a stone wall along the road that may be difficult to negotiate with a 30-footer.
There are several paved sites in a loop that look nice but lack water. These are 3, 4, and 5.
Site 52 should be easy enough to back into and site 43 up the hill a bit seems okay too and has water. Making the turns up the hill toward the site we were assigned first, 46, can be tricky. It's probably best to take a pass on any in the 40s or 50s. Some are extremely problematic, like 46, which was rutted and steeply uphill and requiring a long backup. I suppose those who designated sites as RV sites were thinking of Class A or C vehicles, not trailers. It's one thing to drive into a site but quite another to back in with a trailer when you need room to maneuver the TV on the very narrow roads.
On Friday we had to vacate the North Grove Campground but found a very nice spot in the Oak Hollow Campground about 4 miles further into the Park. We stayed in site 93 and site 94 next to it is even larger and should suit your trailer easily. It's a steep climb up the hill and again the roads are narrow, but large rigs seem to do okay. A 32-footer SOB was in 94 and had plenty of room for the long TV. Site 121 right inside the campground entrance would be a good choice too. Sites 122, 123, and 124 had Class As in them as I recall.
The sites in the Oak Hollow Campground are all paved unlike the mostly dirt sites in the North Grove Campground. That's the good news. The bad news is that there are generally wooden bumpers along the sides that constrict the space and require precise backing. Some of these bumpers make sense to help you avoid tumbling down the hillside or ravine.
We left on Saturday and because we wanted to continue our experiment of camping without hookups, we decided to try the campgrounds run by the Corps of Engineers at New Hogan Lake. We really lucked out with site 120 at the end of a huge cul-de-sac and turnaround. Nice views of the reservoir and the hills beyond. The campground is very popular for large groups with boats and jet skis. Many sites are right on the water's edge but often are sloped downward as you would expect. If we were reserving a site on the water, 127 would be our first choice as more private and flat. Site 125 also seems nice. The prices can't be beat. $16 for Friday and Saturday, $12 other days. Half-price with Golden Eagle (? the senior pass for National Parks). Rosemary will qualify in July!!!
On the way out to Calaveras, we stopped at Lake Camanche South. Clean flat sites overlooking the water, but quite close together and exposed. Also, way overpriced for what was offered, we thought. $36 per night plus $3 per dog (we have 2). In the future, when we want a site overlooking a lake/reservoir, we'll definitely seek out the Corps of Engineer sites. We have the book "Camping with the Corps of Engineers" now and will use it to plan other trips.
Lastly, we also looked at the Golden Pines RV Resort about 4 miles up Route 4 from Calaveras. This place has many long-term residents and wouldn't be our choice except if desperate for a place to stay in the area.
Hope this helps a bit. We thoroughly enjoyed our trip and I'm sure you will too. Reservations will be needed in June, of course.