If your caravan was registered after 1992 then it's easy. From then onwards, all caravans were registered using a universal system (CRIS) and include corresponding identification numbers which were etched into the windows as well as on the chassis plate. See this link for an explanation.
If it's pre-1992 then you could struggle. Most manufacturers used their own numbering system which included the year in it somewhere. ABI were one of the few that did not do this, so the only number is for the Alko chassis on which the caravan is built. As far as I know the numbers were sequential and there is no clue to the year of manufacture in there.
'Highway' is not a model of caravan that I have ever heard of and nothing came up when I typed it into Google. I wonder if it is a dealer special edition, which could make your search even harder.
Alko are still trading and still make the chassis for most caravans, so if you contact them you might just get someone helpful? Otherwise, can you post any photos of your van on here, from which it might be possible to make an educated guess? With a van of that age, I can't see how it will make much difference to the insurance premium if you tell them that you're not sure of the exact year but it's from this period.
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I think the Highway was a dealer special edition based on the ABI Marauder.
Our first caravan was a Mk 1 Marauder which was a 1984 model and I seem to remember the Mk 2 was introduced around 1988 and ran for about five years. They were probably the most popular tourer during this period as they were light and affordable. There were several dealer special editions, usually with plusher upholstery, additional equipment and different paint schemes.