If it came from the manufacturer that way, you're fine. The builder was required to certify that the coach met all applicable Federal Motor Vehicle Safety Standards when he built and sold it. If, however, someone removed the shoulder harness/seat belt combo and put in a seat belt, then you're in a grey area. It's grey because technically you should have both in there and you don't, but in reality very few LEO's know this tiny corner of the law. This is especially true as to how it relates to RV's so if you'd ever be cited on such a technicality, it's very doubtful and I'd be very surprised. Especially if you were wearing the lap belt.
I used to teach child and adult passenger restraint courses to State, County and Local LEO's and very few were willing to write up someone in an RV for seat belt violations let alone someone wearing a belt in a vehicle that "might" have been required to have a belt and shoulder harness. Just not an area they wanted to get into unless a violation was so flagrant they just couldn't overlook it. I was a registered Safety Engineer and set up the Child Passenger Safety Program in Delaware.
2013 Tiffin Allegro Bus 43QGP
2011 Chevy Silverado 1500 Toad