Topo maps are (barely) possible on some of the older units, and not very effective, and impossible on any of the newer units. Can't help there.
When you say "big" touchscreen, how big? The current crop offers 5" screens, but a 6" may become available soon.
There are many feature differences in the current lineup - some include Live traffic (possibly irrelevant for your work purposes) - some include a lifetime map subscription (a good thing) - there are a couple of different mount types out there that might matter if you need to remove it frequently, but don't matter if you can leave it stuck in place all the time - whether you'd be poking at it with gloves or bare fingers makes a difference ... not knowing more about the kind of territory you are in nor whether also you'd be using it for any city driving, etc., it's hard to make a good suggestion.
If we're talking rural-only, fixed (not often removed), then a Via 1535M would do the job pretty nicely. Here's TomTom's link:
http://www.tomtom.com/en_us/products/car-navigation/via-1535-series
The Via 1505M (M for maps) is the one that gets you the lifetime quarterly map updates. Unlike the Via 1535, it doesn't include a slot for a uSD card, but with 4GB of internal memory, it'll hold all of North America without problems anyway. If you don't spot the Via 1505M, the Via 1505TM includes the traffic receiver cable for another couple of bucks. Not sure what they're selling for up north, but on this side of the border, they're less than $110.
http://www.amazon.com/TomTom-5-Inch...FTEW/ref=sr_1_9?ie=UTF8&qid=1336362723&sr=8-9