I have a garmin nuvi 200 sat nav which i fine when used in my car but cannot be trusted when used in a truck. Does anyone know if there is any software i can buy/download for it to make it truck friendly, like you can with Tom Tom’s (or so i’ve heard). Or do i need to buy a new sat nav that is specifically for trucks 
I don’t think you could ever trust any sat nav system 100% when driving a truck.
I’ve got a TomTom and have tried all sorts of different maps & navcores including the latest “truck” software. Yes it does avoid low bridges when you are overheight but the routes it picks by way of diversion sometimes defy logic.
When you decide to leave a route it has picked it sometimes sulks and insists you turn around even if that involves doing so on wholly unsuitable roads rather than adjusting its thinking in line with yours…
To be honest I spend more time shouting at it than making constructive use of it. I wish I didn’t, I wish it worked but its just not as good as a map and mark one eyeball.
It is good for spotting upcoming sideroads though - you know the ones where the sign is on the wrong side of the road and overgrown/missing/vandalised and to that end its useful but you get that with any satnav.
Get a good map - Philips and the AA have both brought out a Truckers Atlas with bridge heights & weight limits on - both very good maps and they make use of the best satnav navcore in the cab - your head!