Well, 5000 miles and seven fantastic weeks of road trip later, we're back in Ireland after a once-in-a-lifetime holiday. So many memories, so many stories.

I just wanted to post on this thread several answers to my original questions, in case anyone ever comes upon this when researching a trip.

1. What is the seating layout when driving in a Class C (or Class A) rv. Do the seats take a standard child seat that can be attached with a seatbelt.
The layout on a Cruise America 30 ft long C class - (Ford chassis) was as Southwest Dave suggested - two seats up front, side facing sofa and dinette seating four. However we had slight problems positioning a baby seat. Trailfinders in Ireland and Cruise America had guaranteed a three point harness for a child safety seat on our rental. However once we were at the rental location, they knew of no such promise, and in fact there was no way such a promise could and should have been made. The only two three point harnesses are on the drivers and passengers seats, and the airbag on the passenger seats couldn't be disabled (or at least it wouldn't be worth disabling) Our carseat didn't work with lap belts, so we were nearly turning around and flying home again (well maybe not quite), when we turned to the nearest Walmart and bought a new carseat that was compatable with lap belts. The baby was put on the rearmost dinette seat, with the table collapsed. I'm not quite sure what would have happened in a crash, but we lived to tell the tale.

The moral being I suppose, is that this could be a dealbreaker if you had to have a chassis attached three point belt to attach the car seat

2. How limited are you in a big RV in the mountains? Do you have to stay on Interstates. Presuming that a big class C would let you go to most places?
Pretty naiive question looking back on it. We got stuck for a night in the Bighorns in early June - 18 inches of snow - and it was pretty hairy in a big RV with not very good windscreen wipers and made it just about. If you were coming onto a road that wasn't suitable for an RV, you'd know all about it.

3. And finally, we're taking a fairly leisurely 6 weeks to get across, driving for maybe 2 hours a day. With the kids, we want to have the flexibility of not having to reach a certain campground by nightfall, but I'm slightly worried about the need to book. In May/June would it just be the popular sites that would be booked out?
Again Southwest Dave was spot on. We used Woodalls extensively, but never booked ahead. The trip was the last hald of May and June, so everywhere was deserted. The one exception was the RV park in Yellowstone (whose name escapes me). We didn't book, it was full up, and we spent the night with no hook up, no furnace and temperatures of 24. Chilly.

I appreciate that there are a lot of variables involved, but how many days consecutive boondocking are realistic in a 30 foot C class, with two adults and three small children? One night - a week - somewhere in between?
We were thrust in at the deepend with a dud coach battery. For the first three nights (night 1 as a virgin RV'er I didn't turn on the hookup, nights 2 and 3 were spent in the Catskills with no hookup), we only had enough power to run a couple of lights for a couple of hours. Generators are obviously a bit of a no no after 8, so we were plunged into darkness fairly early. Once the battery was evetually replaced, life was easier, and we could exist without power, as long as we were careful. As suggested though, our waste tanks would fill up every other day. In reality we only had about four or five non-hook up nights in the trip.

I'll maybe post some other memories on another thread, and will be checking out the forum regularly. Now I can put pictures to the names it means so much more. My specialist subject is probably "The playgrounds of rural northern USA"