Keyword targeting is important. There is no guarantee that just because you rank for "mobile van" that you will also rank for "mobile display van". But because you're ranking #1 for "mobile van", the chances that you will rank for other related keywords in your niche is very high.
Google selects pages to display to its searchers that it thinks are the absolute best pages for those search queries. So you may have the #1 page in your country for "mobile van", but you may not have a great page for related content such as "mobile display van".
So yes, you do need to target the related keywords to your page, or you need to create great pages related to those keywords. If you want to rank for "mobile display van" then you will likely need to have the best page in your country for that topic.
Because you are ranking #1 for the broader topic of "mobile van", it seems that Google believes your site is the #1 destination for the topic. As a result, if you produce great content for related terms in your niche you should do very well.