Voltage inverter in a Cali Beach

[Alfrla]

Last weekend we had our very first trip in our Cali beach. I learnt a few things already :

If you want to shower its good to take soap and shampoo with you
Just because its really hot during the day does´nt mean one thin blanket is enough for keeping two people warm... not at 5 centigrade
Butter melts at 25 centigrade
The inverter: a brand new 1000 watt that will work on the 12v voltage. Thats what they said. The main thing was it did´nt work. The voltage on the plugs is apparantly not a constant 12 volt. I hooked it up directly to the battery and it worked great. Is this a known problem with the 12 volt plugs? Is there anything that I can do?

Adrian