In September of  '04 we towed our 35' triple slide 5er from I-10 to Alpine,
TX via US 67 with our '01 Chevy 3500 Duramax Diesel.  That route takes you
from about 3,150' up to about 4,500'.  More like hills than mountains.  As I
recall it was pretty good two lane road and no problem at all.

While at Alpine we drove solo to Marfa and then on to Presidio.  From Alpine
to Marfa you go from 4,500' up to 5,100' and then back down to 4,700' or so.
I don't recall that being what I would call mountainous, more like high
desert with some small mountains off to each side of the road.

From Marfa you go up to 5,400' or so and then down to about 2,600' at
Presidio.  As I recall the road was pretty good and it was more big hills
than mountains.  I don't think you will have any problem towing from I-10 to
Presidio.

The only other route to Presidio that I have been on is from there east to
Lajitas via SH 170, then on to Study Butte and up to Alpine via SH 118.  I
would not recommend that at all as SH 170 from Presidio to Lajitas has some
15% grade, numerous low water crossings (good to hang up the back end of a
trailer on) and isn't really meant for heavy vehicles.

Ron

Do you mean Valle de Allende in the Jiminez-Parral area of Mex 45?

I drove to Chihuahua city from the Buffalo area in 1998 via
El Paso/Juarez and Mex 45 with no problems.

Went there from N Orleans in 99 via the same route you propose,
crossed at Presidio/Ojinaga.

Both times I was driving a compact car.

Mex 16 from Ojinaga to Chihuahua city was OK but it was very windy.
Best be aware of that, towing a trailer.

Also Mex 16 had a number of vado/ford warnings which Mex 45 didn't.
Could be a problem if it rains.

You won't lose much time by taking the El Paso/Juarez route and it
might be safer.

If you survive I-10 to Beaumont the worst is over.
Larry