Frank, Depending on what time you come through Hoover Dam may not be your best choice. They may want to inspect your trailer(If they will let you across with it) and contents and the traffic can get bad on the weekends. I bet your coming through Flagstaff then through Kingman AZ. Do yourself a favor and once you hit Kingman go through Laughlin NV and bypass Hoover dam. It may just save you one hell of a bar tab and it's not much farther but usually a hell of alot faster.
http://www.usbr.gov/lc/hooverdam/crossingguide.pdf