The Rapido website is wrong. The figure they are referring to (75kg driver, basic equipment etc.) is called the Mass in Running Order (MIRO).
The maximum allowable weight of the vehicle fully loaded is called the Maximum Technically Permitted Laden Mass (MTPLM) or sometimes the Maximum Authorised Mass (MAM).
So, MTPLM minus MIRO = the practically usable payload after accounting for fuel, gas, water etc. (But you still have to make some adjustments, e.g. most of us are over 75kg!)
Unladen Weight as Kev says is the van basically empty. It is of little value to a motorhomer, other than to calculate the absolute payload. It is also the figure used to determine whether your maximum speed on a single carriageway is 50 or 60mph, and 60 or 70mph on a dual carriageway. If the unladen weight is over 3050kg, the lower limits apply.
I have found it really hard to determine the unladen weight of my Hymer (other than with a weighbridge) but from the catalogue, I can read the MIRO and then adjust it for extras like awning, additional battery, bigger engine, auto box and my real weight etc.
Hope this helps rather than confuses.
p.s. have you looked in FAQ section? I cannot remember whether there is a paper on weights in there, but would guess there is.