LPG gauge

Our gauge on the dashboard for the underslung tank has 4 green dots. It has taken about 21 nights for when the ignition is first on it goes to red then goes to one light and driving sometimes two lights.  I realise that is due to the level on the van.  We were then looking for somewhere to refill and the person pitched next to us said they still have about half a tank when this happens. a full tank is around £6 and surely after 21 nights nights it can not still be only half empty!  Usage is fridge, hobs, and oven maybe 3 times a week. We dont bother with hot water as kettle suffices.  
Question I guess is, how accurate is the gauge (when level!) or is it a bit like the water tanks hit and miss accuracy?

You have the same gauge as me (later ones have 9 lights and there is a recent 'factsheet' covering that) . When I start engine I get the red light first then if I wait without moving van, the first green lights ,then the second and sometimes the third or even fourth if I've just filled up. You get used to what each light means. With mine if I only have 2 green lights then I'll get about 6 or 7 litres in. If only one green light then I'll get about 11 litres in. Since lockdown we've done 14 nights and the gauge hasn't moved from 3 lights (although it does seem to take a while for that third light to show) We have only used sites with hook up so the gas is only used for the cooker. We use the oven every day for maybe an hour, everything else powered by electric.

Hi  Lancy   yes your right  ,the  LPG Gauges are  not very accurate at all

Just done 5 weeks in france we know the tank lasts that long even with BBQ'ing with the occasional night on electric.

Thank you for your helpful replies and it saves taking a divert to fill up today and can just call in at our local Morrison’s when we get home on Tuesday.