As far as the law is concerned, the two people in the front seat need to wear belts. The articles quoted are so much BS, as the section of the RV behind the RV is not considered a vehicle, it's a home. This is why search laws differ in RV's and cars (trucks, vans, etc.)
This does not mean that using them in the back is a bad idea, or that they should not be used, but there is NO LEGAL REASON to use them. Trying to tell how the laws apply is complex, but I'll give you the VERY short version.
The FEDS (DOT) wanted all states to pass seat belt laws and they didn't have jurisdiction. In order to "encourage" states to pass the "proper laws" the feds withheld fuel tax payments due to be refunded to the states as a function of the amount of federal tax we all pay when we buy road fuel. Big bucks. At first these were front seat only and then they started working on all positions. Going along with that the Federal Motor Vehicle Safety standards were revised so all manufacturers installed and tested belts and anchorages according to the same standards. This required a large number of full crash tests and many more sled tests. Needless to say, BIG $$$$. Based on the number of documented fatalities and injuries experienced in RV's it was determined that the cost of testing multiple seating positions in the back of RV's, did not mandate seat belt testing in the rear. Of course manufacturers objections were also huge. Regardless, some manufacturers elected to put belts or belts and shoulder harnesses in the back based on "feel good" emotions. Some are worthwhile and some are downright dangerous, but they are there in some RV's. Bottom line, no standards, can't have a law because effectiveness is always in question.
By the way I taught this subject to state and local police, medics, nurses, firepeople, and many others for 8 years. I was the first certified Child Passenger Safety Specialist in Delaware, as well as being a licensed safety engineer