I have used
Proclip , for mounting cell phones and gps in car and MH. They make an assortment of brackets specific to each vehicle , they mount in various areas of the dash or on console. You can then get different mounts and extensions for the various monitors.
The dash cam is usually mounted high on the windshield, behind the rear view mirror. Its there to give evidence or your good driving skills and that of the poor judgement of others.
Could the monitors for your rear observation camera and back camera be combined into one monitor with 2 or 3 inputs.
Ken