The difference between the two in the surejust site seems to be the pressures. one seems to be pre-pressurised to a certain value; when buying the same device for the alternate purpose it is a different pressure (and you can set/change it yourself anyway (there is a Shrader valve on the bottom usually)).
And as you have found, it is just the the location that defines what it is used for. With the very limited pressures on a European/UK Motorhome where there is no pressurised water source, a basic water pump and a small hot water tank, I doubt if it matters much which is used (and if one is used as all).
Personally I think it is a matter of installing as per the heater makers recommendation. Why try and re-invent things for little or no additional benefit?