2nd AC unit

A lot depends on how your rig is set up but you can usually either hook it to the ductwork, or use an air distribution box with either mechanical controls or wall mount electronic. Many rigs use a multizone thermostat, with control lines run to the front vent, or you can install a separate thermostat.

13.5K low-profile Penguin has served us well, even in Arizona last July.

We had a second unit 13.5K unit installed by the dealer.
Plus a Fantastik in the bathroom which is on the top level. This is set to blow the rising warm air out of the unit.
For a little overkill we have a desk top fan sitting by the bedroom door blowing cool air back down the stairs.
So for much of the time we only have one unit on... but for max cooling we turn both on.
All is good for us.
GPG

Our 2nd A/C is a 13.5k and, along with the 15k rear unit, is sufficient to cool 5er fairly quickly after a hot day of travel.

If you are borderline with a single Air/con, a second unit, even if the smaller 13.5btu should easily keep up.

One thing, with the single unit, can you shut off the vents to the main living area at night? With our old unit, by opening and closing vents, we could pump the cold air to the area we were in (bedroom at night and living room during the day). We would do it on very hot days and that let the single unit keep us comfortable. Unfortunately, out new rig doesn't have vents that close, so we have to figure out what to do with it.

Had the 2nd unit installed by the dealer. They put in a 13.5 and it worked just fine. Had the trailer back in 1 day.

When I did this to ours, there was a sticker on the junction box cover where the new AC was going, listing max amperage. So I had to stay under that. I put a 13.5k unit in, and that is big enough.

Mark