Just look at the RV industry stats for shipments of new RVs of the type(s) you are interested in and then figure a reasonable annual attrition rate (maybe 3% or 4%). That would get you pretty close to how many are still registered and road worthy. I am very skeptical that even if it is approved for offshore use, it will automatically transfer to vehicular use. There are big differences, one of which is Yachts generally are under power for a big percentage of their usage, where RVs are primarily stationary and powered down the vast majority of the time. It will take a long time and a lot of heat for an RVs exhaust system to vaporize 100 gallons of grey water and a equivalent amount of black tank waste. I am also skeptical of how well a black tank will drain drop by drop, since they are designed to be evacuated quickly.
