It varies from state to state, and is not always the same laws as for cars (though I suspect that's the more usual case). In Florida, for one example, the usual seat belt law does not apply to motorhomes.
Be aware that booster seats (i.e. ones that do not have their own belts attached to them that secure the child to the seat) should not be used on seats without shoulder belts. They work by positioning the child and the vehicle seat belt properly in relation to each other. With no shoulder belt, they can't work properly.
I doubt any child seat is rated for use when facing sideways, and probably few for a rearward-facing seat.