A vehicle's payload refers to the total weight of everything you take with you in your van (including yourself and other travellers).
A full tank of diesel, a full tank of water, and cupboards stuffed with things you don't really need all add to the payload.
Every motorhome will have a safe payload limit (usually stated in the manual or stamped somewhere on the van itself). The danger is that if you go over the limit you're breaking the law. And if you have an accident and are found to be overweight, your insurance will be null and void.
Dive into the details about motorhome payload in our blog post here