BigJim gives good advise. Dual antennas 'look' good but making them work requires an impedance matching harness (to get back to 50 Ohm feed again after paralleling two 50 Ohm antennas) and the combined pattern is probably in the wrong direction anyway.
Top center is the best place. Larsen makes some really good base loaded whippy whips. If you need a source check out Ham Radio Outlet www.hamradio.com. The model that covers 10m ham band can cover the CB band by adjusting initial cut length. My Larsens have survived trail use for years.
You can have a long row of antennas down the center line of that bus!!!
U-1300L Unimog with 10' Alaskan camper, OM-366A turbo-Diesel, fast axles, overdrive, Werner 12k winch
Various other mogs for work and play.