If the seat physically won't go rear facing in the motor home, or you have to "rig" it to do so, I would think he'd be safer forward facing. Besides, I'm pretty sure the rear-facing recommendation is for the standard passenger vehicle. A motor home is a different ball game.
Definitely get your hands on a portable DVD player and bring along all the Baby Einstein videos. I usually pack 2 outfits per day simply because it's a pain to do laundry on a trip, and baby clothes are small, so it's not like you don't have the space. I typically pack a separate bag for the kid(s) because their stuff tends to get lost in the grown-up things. I just bring a whole package of diapers and a whole container of wipes. Wipes are used for about 100 different things besides diaper changes when you're on the road! Lots of hand-wiping on-the-go. I also pack a small duffle of toys and books.
The most important thing to keep in mind when traveling with a baby/toddler is that it's best if you're not in a hurry and "trying to make good time." It may be hard for your in-laws to understand that, but even if you just pull over at a roadside rest area, it's worth it. When we traveled with my daughter at this age we pulled off the road at one of those places and just let her walk around for a few minutes, got a fresh diaper on her and she was fine for a while.
On a recent road trip with my 2 and 4 yr old we literally stopped every hour on the way to our destination (made the trip from Denton to Killeen take forever!). By the second day of travel they are usually used to the car and we can drive for longer intervals. The first day out is the hardest. By the time we head home, they are great and sleep most of the way.
It's also good to note that when you do stop, EVERYONE should try to take care of business, get food, stretch their legs, whatever, because when the baby sleeps, there's NO STOPPING :-).