There are no standard weights for 24, 26, 28 and 30 foot trailers. Every model trailer is different. My Coachman Captiva 265EX (26') trailer is 5500 empty weight. I am sure you could find a 24' trailer that is heavier and a 28' trailer that is lighter. In general a trailer with a slide out will most likely be heavier than a similar trailer without a slide out.
I would expect you should be able to find a travel trailer that your cargo van can tow safely without having to go with a tent trailer.
There are many more weights you need to consider when finding a trailer, someone will chime in shortly with that information of search for one or the many similar posts.