Hmmm, sounds like a tough choice.
I am not very familiar with motor homes, and without seeing the vehicle in person I don't think I could give you better advice. I would highly recommend seeking out a certified child passenger safety technician in your area, preferably an instructor. They can do a free inspection, and may have some suggestions for you upon seeing the motor home. Please visit the "Free Carseat Inspection" link under the top banner to locate one in your area.
Unfortunatley, I suspect it is probably the case that many children in motor homes often ride unrestrained or improperly restrained. Unrestrained children are not uncommon in normal passenger vehicles, either, and that is a big reason why motor vehicle crashes are the number one cause of death for kids 1-14 :-(
Darren