As above, and this is also valid for oldtimer trucks from abroad, Belgium truck owners with an oldtimer registration are exempt.
This is for all trucks above 3,5 tons, worst thing, you have to hire a special box, an OBU, on board unit, that is a sort of a GPS, that records and sends out where you're driving etc. this is coupled to your credit card, and automaticly charges for what mileage you're doing! (15-30 eurocents a kilometer), failing to do so get's you a 1000 euro fine!
Not a good thing! don't think they'll have a 6volts version, as all modern trucks are 24 volts.
Please check it out before traveling guys!
here a link in English I could find, nothing about foreign oldtimer lorries though:
http://www.viapass.be/en/about-viapass/viapass-for-hgvs/
Cheers,
Lex