Sorry, Klastri, I forgot to mention that heavy lift helicopters can also be used to air lift portable sleeping cabins, administrative offices, and other support facilities and generators to remote areas along the border without having to build the roads you keep mentioning. They also can be used to quickly ferry workers to and from the job sites.  The government may not own much of the land, but is very capable of negotiating to rent or lease it or take it by eminent domain.   A lawyer would know that. 
One more thing.  With regard to filling in trenches along the wall, the plan is to hire Mexicans to do it by hand with shovels on the Mexican side, even though it will mean they will be on a small stretch of U.S. soil when they do it.  I understand there already are hundreds of Mexicans who have signed up for the work which will pay the minimum U.S. wage and allow them to return home each night to a life of luxury.  You see, I have thought of almost everything.