I've only had light colored coaches (and would not get a dark one), so I can't compare. However, the mh in my sig only has one A/C, which concerned me when I bought it. It is prewired for a second A/C, so thought I could add it later if needed.
I spent two months, July and August, in Branson, MO a few years back with temps over 100 every day during July and mid to upper 90's in August. My single 13,500 btu A/C worked just fine! Since then I have been in the South (Alabama, Georgia, N. Carolina, etc several times during the summer and have had no problems keeping the coach cool.