Welcome to the forum ceegeemm.
A general overview:
"Newer" Chevy vans will be based on either the 1996–2002 Chevy platform or the 2003 to present platform. More passenger seat legroom from 2003.
"Newer" Dodge vans will be based on the 1994 to 1997 or the 1998 to 2003 platform. New instrument panel from 1998.
Note that sometimes a RV will have 2 model years. A 2004 RV part on a 2003 chassis for example.
If you don't know much about vans then a newer one will probably be better for you. Things like airbags etc., could be important.
A mechanical inspection is advised. A test drive is a must. Check the age of the tires. A new set of tires could cost you $800 to a $1,000.
If you are particularly interested in used Roadtreks then be sure to visit their website. They have brochures dating back to 1978 available:
I'll rotate my wheels and change the oil myself etc., but I really don't know that much about the mechanical side. So hopefully, other members will contribute here and steer you in the right direction.