As Isaac points out, the UL has a testing specification for the designation of "RV Use", UL 2034 for CO detectors and UL 1484(RV) for smoke detectors. However. RVIA standards do NOT require that detectors used in RVs meet those UL test specs.
The use of the acronym "RV" in the UL specs appears to be somewhat broader than our scope of interest, likely including salt water yachts, ATVs, etc. The testing includes a 48 hour exposure to salt spray, 30 days at 150 F. and 3 days at -40. Few systems in our RVs could withstand those extremes. I'd hate to see what a typical RV would look like after 48 hours in a continuous salt water spray.
The testing for the UL RV Use designation is expensive by itself and making the product capable of passing it adds more expense. That's why you see so few with that UL-approval labeling. The detector doesn't detect any better - it just holds up longer in extreme conditions.
There are, however, detectors that are NFPA 1192 approved, and that's the important measure. The RVIA also has a list of approved detectors that their manufacturers use in new construction. Here is one example:
http://www.brkelectronics.com/pdfs/2016/04/15/75e7f29c.pdf
Re the mounting height: CO mixes readily in air and CO detectors work at most any height. LP, though, is heavier than air and LP detectors must be located near the floor. Thus a combo LP/CO unit must be mounted low, near the floor.