You can certainly run two 13.5kw roof air conditioners off of normal 30amp RV service... but most people don't realize that their battery Charger/Converter, electric Water Heater, and other things are also running in the background, which can trip the breaker, making them think that it's not possible. Sometimes campground 30amp breakers are also weak from overuse and trip too quickly.
We've run both ours many times in the hottest and humid times with no issues, but I turn off the water heater, turn down/off the battery charger, and monitor our other outlets during that time. I turn off one a/c unit for the few minutes when needing the microwave, etc.
But, having said that, it makes sense if adding an additional 13.5kw roof air conditioner to keep it simple and run it's own 20amp extension cord pigtail so that you can use the additional 15/20amp outlet at the campground. Generally, the a/c unit can operate from a 15amp outlet as long as the fan and compressor don't start at the same time. Most newer a/c units separate the two timings, making it an easier proposition.
Even if the campground didn't have the additional 15/20amp outlet, you could plug it back into one of your outside outlets, or one inside the coach, on a circuit that is not being heavily used, and run both thru your own 30amp connection.
You could add the a/c unit on the roof, and run the pigtail/extension cord down to the ground, versus trying to handle the wiring inside the coach. The wiring could just ride on the roof, wrapped around the a/c unit, while traveling. We do our satellite antenna the same way.
I'm sure the 'easy start' products might give you more confidence, but I also think it's very possible that you don't necessarily need them.
Your generator's ability to run both will depend on it's Amperage output. A typical 4kw gas generator will have a ~33amp output, generally to a 15amp and 20amp onboard breaker set, feeding both back to your Main panel. If you are cautious about your other usages, you may find that you CAN run both at the same time while on the generator, though you may have to trip your Converter's breaker in order to keep it from also trying to charge the batteries at the same time, which may also leave you without lights and other 12v items during that time.
: ) give it a try...
(of course, there are also portable a/c units that can plug into a regular outlet and the unit is on wheels and can sit inside the coach anywhere you want it to, though it may need a vent or drip line out a door or window)
We use a lot of small 120v fans around our coach, too, as air movement is really just as important as the temperature itself.
'14 Thor Palazzo 33.3 compact diesel 100k+miles since '14... US, Alaska, Canada...
AND a '14 Gulfstream Amerlite
AND a '07 ForestRIver Rockwood