Inverter Trouble - They keep blowing!

Hi All. I run a small business from a caravan & have been using inverters to power it, however I have killed 3 decent ones. They just stop & come up with the fault light, no fuse blowin nothing. The last one was the size of a house 800w continuous with a big fan etc. Lasted no time at all ( a day ). I have had the electrics checked out in the caravan and found nothing. The last one was running the following when it went: a 15w light a 36 w strip light a set of weighing scales ( very little power) a small radio a phone charger. It had been runnig just the scales / Radio & phone charger during the day - only had the lights on for 5 mins when i got dark when it went. It inverter wasn't hot at all. This is typical of the load. I have a flashing rope light I put on, but only tend to use that with the generator, as although one of the inverters did run it b4 it died ( again well within wattage) the battery - a 100ah leisure battery - was eaten up quite quickly when it was all on & the inverter did get quite warm, so never really risked it. Any ideas? I am running out of ideas! cheers all. Mike