The 'Watchman' campground in Zion is the place to stay, it's convenient for the lovely town of Springdale and you use the nearby free shuttle bus system to get around the park. We booked a riverside site which was very nice. I think I mentioned previously, the 'Pines' campgrounds in Yosemite are centrally located in a wooded area, really nice. Go to the nps.gov site and se when the booking window opens and be prepared to book all NP lodgings straight away it does. I can't emphasis this enough, there are limited RV spaces and they are hugely popular !! It doesn't take to much to get away from big crowds in the park but you can expect a lot of people during the height of the season. I would suggest you buy supplies before heading into the parks and 'eat at home'. The eateries often have long queues, are overpriced and pretty average, besides you can't beat some good grub around the campfire with a couple of beers. The one exception is you may enjoy an evening out in Springdale, an easy walk from the campground.
On the way to Zion there are a couple of stops along the Mt Carmel highway, Checkaboard mesa and the canyon overlook trail but both have limited parking. You will have to pay $15 to go through the tunnel and will have to wait for a little while for them to halt traffic coming the other way which is normal for larger vehicles. It's a fantastic drive but make sure you are at the tunnel before 8pm or you won't be allowed through until 8am the following morning.
Between Zion and Tonopah you will drive the 'ET highway' (NV375) where you will pass the The Little A'le'Inn which is worth a quick stop for a refreshment and to look at the 'other worldly' photos all over the walls. Where NV375 meets US6 you will see some derelict buildings immediately you turn left, which is known as Warm Springs. It's quite a cool place to stretch the legs for 20 mins or so and have a look around. You can see the old stage post coral, an old generator room, old car wrecks and a man made hot tub/pool. If you walk up the hill following the sulphur trail a little way you will come to a natural hot spring bubbling out from the mountain that feeds the pool below. If you see some dust being kicked up in the distance it will likely be the Area 51 guards patrolling the area.
As for the RV, driving is quite straight forward, just give yourself time to adjust to the extra weight and size. You will soon adapt and be much more sensitive with steering and braking input etc. The back is wider then the front with a slightly wider wheel base so you have to park alongside a large kerb carefully and you need to remember you have an overhang at the back. So when pulling away from fuel pumps, parked cars by your side etc, steer out cautiously to avoid the back swinging out and catching something. While on single lane roads, especially slow twisty ones keep an eye on the traffic behind. If you are holding up traffic try and find a pull out and let them by rather than hold them up, you will be rewarded with a friendly toot of the horn and a wave and it takes the pressure off.
Oh yeah, nearly forget. At the first park you visit purchase an annual pass for $80 at the kiosk, visiting GC, Zion and Yosemite individually will cost $100 and it saves the trouble of digging around for money at each entry point.