series is best for A stationary installation in which both panels are exposed to the same amount of light, as on a home.
But a camper may have one panel full Sun, the other part/full shade. If wired in series, the shaded panel would pull down the output from the full Sun panel. In parallel connected panels the shaded panel does not hinder the power of the other panel.
2000 Cherokee 29BH with 6V batteries, LED lights & 400 watts of solar power, flipped axles and raised. 2007 Tundra 5.7L DC-LB with lots of mods. C-co, 8/158th AVN Maint.