Ive been looking in to this a bit after reading your thread..
Like most people Ive been able to use M2 campaign map textures.
Ive found a converter tool which will convert textures to dss files (In truth this wasnt hard to find on the M2TW mod workshop pages)
http://www.twcenter.net/forums/showthread.php?t=74690
However looking at the M2TW textures they are very different to RTW
The battle map textures in M2TW are done in two parts a macro and a micro texture, the macro texture is like a rough coat of paint and the micro fills in the fine detail close to the camera viewpoint. In contrast RTW just has one texture, when I tried to use the M2TW textures they tiled quite badly and didnt look good.
As for the grass the textures are also different
So its certainly not possible to use as they are, however I think that they could be used to make more RTW texture. Even then there are problems if you look at the vanilla grass its virtually white this allows it to blend with the ground texture. The M2Tw textures and for that matter Proms dont do this.
I'm currently playing with modding the Rtw ground textures, and will have a go and making some different grass textures (have a read of this about grass textures http://www.twcenter.net/forums/showthread.php?t=60472)
Ive not got around to looking at the trees and such, but it wouldnt surprise me if that worked a bit differently too.
If your serious about this I would suggest having a look in the M2TW mod workshop, see if anyone has modded the vegetation and ask how or search of a tut then compare how its modded in RTW you might find there are similarities. From what Ive seen some elements of M2TW could be brought into RTW but even that will take some work I dont think there is a quick fix.