I would be concerned about safety too. Then I thought some more and want to give you a different perspective. I'm 65. We did all sorts of things when we were kids and no one that my large extended family knew or knew about was hurt. Yes, I've heard of kids being hurt in car accidents because they weren't wearing their seat belt but when you think of the number of kids that are not hurt it doesn't seem so serious.
As far as I know there are no laws about using seat belts in a motor home. I think that if there were a perceived danger motor homes would've been added to the law.
I've seen a lot of vehicle accidents as a police officer and been in a couple myself. It is usually the smallest car that suffers the most damage. You have more of a chance getting into an accident with a car than with a motor home.
When you think about it, a motor home is huge in comparison to most vehicles on the street. If, and it's a very very slight possibility, your kids would be bounced around and possibly get cuts and bruises but they would be very very unlikely to suffer serious injury. This is because there is a lot of space that they are bouncing into.
The size of the motor home prevents the motor home from sustaining much damage in the usual crash scenerios. If you were in an accident your kids would not be shoved into the back of the seat. When we are in a confined space we hit inert objects with more force. Yes, the seat belts do prevent some serious injuries but kids are seriously hurt once in awhile even while correctly buckled into their car seat.
As I reread this I am thinking, "gee, this is scary." I'm not trying to scare you. I think I'm saying that you are taking no more of a risk with your kids safty in a motor home than you are in a car. The chance that you are in an accident in either one is very low, anyway.
If I were you, I might have the kids sit on the floor. Bring toys, without sharp edges for them to play with. That way they won't be bounced onto the floor if the driver has to stop suddenly for any reason. A sudden stop is more likely to happen than an accident.
One of my friends who was my partner at work took a trip in a van with their toddler child. They took the seats out of the middle and put a play yard with the mesh sides in the middle so their daughter was somewhat confined but still would not hit a solid part of the vehicle if they did stop suddenly.
To keep the kids entertained I suggest getting a DVD (or VCR) and several movies. Actually, I think that you would have a better trip if you are all in the motor home. There are many more play options. Being confined to a car seat is difficult for kids. If they are fussy and unhappy this can be a distraction to the driver.
Don't use the car seats in the motor home unless they are securly fastened to the frame with a seat belt. I think they only have seat belts for the driver and passenger to his left.
Choosing the safest way to travel is complicated because each situation has it's pluses and hegatives. My feeling is that I cannot control all circumstances. If I use reasonable caution I and my loved ones will be OK. It really is a small minority of people who are seriously injured when you take into account how many cars and how many miles they travel. I'd take only the motor home.