Most of the border is extremely remote, areas around here can be over 100 miles from a small town 150-200 miles from a city. Logistics make it hard to utilize utility workers or others. Most areas of the border are very sparsely populated rarely visited by utility workers. Rare to find contractors or service companies willing to do work in the area. One of the reasons the virtual fence was not successful is it is nearly impossible to get contractors to go to the border to service things. In my area when service people are sent to the border the drive time alone from the nearest city of any size to and from takes up most of an 8 hour shift. No motels or restaurants so if they have to return the next day they have to go 100 miles one way to get a motel or meals. No services available, not even basics. The issue with tunnels is they are nearly always in populated areas, where there is basically a city on both sides of the border. They are often unnoticed because they appear to be part of the storm drain or other infrastructure which makes them hard for anyone to detect.
I say put basic boot camps all along the border and there will be plenty of access — and firepower — to defend it.