We've always had 1 AC unit. But in our class C, last summer, we experienced extreme heat and I looked deep into a 2nd unit. Controls on the unit, and I'd add a male plug to a storage bin, so that I can plug it in at a campsite in the 20A outlet.
Our issue was keeping back bedroom cool -- it would hit 78 to 81 deg in dead of summer. And rest of unit was only 77to 79. Add to that 4 of us plus 2 dogs...so lots of heat.
HOWEVER, I ended up trying a $9 camco roof vent cover. Yes, we have the external vent covers, but this is soft with reflective material on one side. Fits in the vent on the inside...and wow...it made a huge difference. We have 2 vents and a skylight...so lots of heat loss/heat gain (depending on season). Sure, another AC unit would be even better...but for now, I saved on the cost and pain of install with 3 covers at $9 to $11 each.
Note I looked heavily at eTrailer for the AC unit -- better cost, all the parts you need, and tech help.