Our fifth wheel here in the UK, and the one we owned in the US both had drop through domestic type toilets with the waste going in to a black holding tank. The obvious advantage with an RV is the tank is higher than the ground level, so emptying only requires gravity. On a narrowboat, as already mentioned, emptying one tank (cassette) in to a larger tank becomes a pump out, or requires several people to carry a heavier tank to a dump point, and then require them to balance that over said dump point whilst it is emptying. Having seen the state of many elsan points where people cannot empty cassettes accurately, I dread what mess this would make.

In post 85 by IanD, he asks whether a boatyard can do gas-work and have it signed off by a gas safe registered engineer (para 2) I cannot state current regulation having retired 7 years ago, but at that time it was illegal for a gas safe registered engineer to sign off a new installation completed by a non registered installer. This technically meant that even if both were employed by the same company, unless the GSR engineer did the installation, they would not be able to sign it off. A non registered engineer couldn't technically hang a boiler, run a length of gas pipe or put a cooker in it's place. Prior to gas safe, CORGI only required that a company have 1 engineer with qualifications for the entire company to be registered. Once Gas safe took over, every engineer employed on gas work had to be individually qualified. If an engineer left one company and started working for another company, they would not be able to carry out any gas work until a new registration was issued with both the companies and the engineers name on the card. This is the problem when rules are made by people who have never actually done any relevant work in the field they are regulating (eg, any government minister), and rely on advice given by sources with a vested financial interest. Any engineer with several years hands on practical experience, who has sat in a classroom renewing their gas safe certificates could probably tell many stories of other engineers attending who you wouldn't trust to blow up a bicycle tyre. In the defense of the GSR engineer who puts his reputation first, until regulators, law makers and those in the judicial system that try cases in court all have knowledge of what they are actually doing and make the regulations so that ambiguity in interpretation is a thing of the past, he must certify as he sees fit. His livelihood depends on it. Until then, differences of opinion even within qualified engineers will always happen.