First night out, one issue ???

IDK about yours, but our thermostat reads the temp we set it at, not the actual temp inside. I can set the thermostat at 74 and the A/C will start to jog on and off once it gets to that temp inside, then I just bump it up a degree or two until the A/C settles in and it keeps it pretty much okay inside. I have had more than one dealer tell me that no A/C will manage high temps well in direct sunlight, but that 8000 BTU unit should handle your 19footer okay in the shade; our 10K handles our 25 footer pretty well and ours is not really significantly bigger than yours either way. Unless your thermostat reads current temps and set temps (like the one that might be in your house) it is showing set temps... On that note, post up a picture of your thermostat for the forum if you can.