A ? about Inverters and Voltage Outputs

With all the electronic gadgets we all now collect to take on trips with us I thought it more sensible to purchased a inverter to charge all the mobile phone batteries , video batteries , digital cameras etc etc as they all come with a 240v ac charger. The model I purchased is a 500watt with a 700w surge capability dual outlets modified sine model . It preformed well for our six weeks through outback NSW , SA and Queensland doing all the jobs it was intended . I also used it every day to run an electric shaver and of a night to power a standard 13 watt enegy efficient globe which was super bright as any who has travelled severly corrugated roads knows that mantles have a life span of 2 corrugates . Upon returning home and going back to work I was telling the electrican I was working how well the inverter worked and remarked how much brighter the 13w globe was running through the inverter than what it was when plugged into a 240 volt outlet at home . He said that this should not be the case as it should be the same . We then tested it with a voltage meter and found out that the voltage output was about 300 volt . A ring to the supplier and they said if that was the case they would replace it as it was faulty so it was packed up and sent to them for shipping . The unit was checked and sent back to me with a letter saying every thing was ok as they had tested it and with 12 volt input its output was 254 volts but when they increased the voltge from 12v through to 13.5 volts the voltage output went from 254 volts through to 270 volts but they said this is normal .
My question is this, a battery in good condition will have a voltage output of about 12.7 volts so the output will exceed the 240 volt straight away , a battery with a 12v output is about 25% charged so pretty dangerous stuff running around out back . The unit also says it can be mounted permanatly to the vehicle but it obviously wouldn't be usable while travelling as the alternator is putting about 14.5 volt input back into the battery .
I am worried about using it again on the cameras etc as the higher voltage input may damage the electronics . IS there any way a voltage regulator can be fitted to the input or out put or do I cut my losses and throw it away and start purchasing 12 volt adaptors for all my gadgets . Any help greatly appreciated .
Note : we plugged the voltage meter we were using into a 240 volt outlet and it read 240 volt so the meter was ok , the voltgae output of the battery we were using was 12.45 volts but the inveter still showed an output of 305volt to 311 volt .
What the !?
Cheers
Ken