Best way to lift/stiffen the rear end

[vespa 494]

I,v just bought a lightweight caravan which i tow with my 2006 600kg 1.9D van, it sits abit low imo, whats the best, most cost effective way to lift it, bigger wheels, stiffer springs, spacers, or is there some other way, i can manage as is but it would be better slightly higher i think, cheers.