Depending on what wattage your kettle is , if a normal domestic 3Kw type then that draws 13 Amps and the microwave will draw at least twice its rated power initially on start, so a 800w unit will draw at least 1600w, or almost 7 Amps.
The van is limited to a max of 16Amps but you were probably drawing 20Amps, so tripped the MCB or the site supply bollard trip (most likely the site bollard as that is the one that normally trips before the van.)
What should happen when hooked up to mains and the battery charger is switched on, it should keep the battery charged, but that itself is drawing power as is anything else using mains power such as water heater, fire etc
Also, not all sites have 16Amp hookups, some are limited to 10Amps so you need to ask when arriving on site and work out what you can have on at any one time.
Moderator Note: I put RCD instead of MCB which has now been rectified.
Thanks to Bertiboy for pointing out my faux-pas.