235/65/17 is a popular size, especially with AT tyres.
It will be slightly larger in diameter than your 235/55/18, by nearly 22mm in total, giving you slightly more ground clearance while also reducing the arch gap.
Have a look on "willtheyfit.com" to compare sizes, if you haven't done so already.
If your van was on 16" wheels originally, the 235/65 may cause the speedo to under read a little. If it originally came with 17's (coded for a 17 inch chassis) then the 235/65 should put the speedo bang on.
18's... not sure!!
If you search the forum for "235/65/17" there will be lots of info and pictures of what you are planning to do!!