If you run an inverter that feeds a 80Watt heater, roughly speaking you will be drawing 100-120 watts of power from your battery, this is because Inverters are not 100% efficient, especially when running close to their capacity. 120 watts at 12 volts is 10Amps. You say that your van battery is 500amps, do you mean that it is a 500Ah capacity, or that it can supply 500 amps? Just divide your battery's capacity by 10, the amps that the heater/inverter draws. So therefore if your battery's capacity is 120Ah then it would last for 12 hours before the battery would be flat. That would be an ideal situation, in the real world your battery would be flat quite a bit a lot sooner and you would have to push your van to start it !!
jeff i checked my battery and it said it was 600 amps and 60 ah, according to the calculations will my battery power it for 6 hours then? and if it does how many volts will be left in it do you know? i'm lost when it comes to amps, volts and ah 
many thanks
Sorry easy clean Just noticed your topic,
your calculations are correct, but I would strongly recommend you don't use your van battery for anything other than what its meant for.
as been mentioned before inverters are not 100% efficient and neither is a battery, if you keep using your van battery for the inverter, you are going to drain or deep cycle your battery on a regular basis and there not made for deep cycling so within a short time period you are going to have to fork out for a new van battery, but not before the inconvenience it will cause you when your on jobs and your battery wont turn the engine, also first thing on a winters morning the amps drawn from your battery is huge,
Is there no way you can run an exstension lead to your van?? If not then I would look into the options of insulation of your tank and pumps, so much cheaper and more reliable.
I can only advise you on your batteries but there are plenty of guy's on here who will advise you on insulation.