If it says that it is prewired for another air conditioner, that means that the wires to carry 120V power to the unit are there and should probably be insulated on the ends and may have a separate circuit breaker installed for it, but in some cases the wires are there at both ends but no circuit breaker is installed, just a place to put one. If the power cord is 30A then using two of them would be pretty limited but by how much would depend on the size of each air conditioner. The most common sizes are 13,500 BTU and 15,000 BTU but Dometic has one that is 10,500 BTU and Coleman has one at 11,000 BTU. The smaller units will draw about 7A when running, mid size 8-10A running and the 15k usually need 10-13A running and any of those will need several times that when the compressor starts. A circuit breaker will only allow a continuous load of about 80% of the rating or 24A. On a 30A supply, if we assume the a/c that you have now is 15,000 BTU then you would have about 15A remaining when it is operating and it would run a second air conditioner, if nothing else was operating, or with only small loads. Keep in mind that your 120V/12V converter will demand at least some current all of the time when plugged in so that probably means that you could only count on about 20A total for other things.
It is not true that all RVs with 2 air conditioners or 1 and prep for another are wired for 50A service. It has become pretty common for those with 2 a/c units to have a 50A power cord but it is far from the standard and the size of your 30A cord does not say that your RV isn't prewired for one. You can tell if it is by checking your 120V distribution panel for a second a/c circuit breaker or provision for one with the wires there. There would also be wires for power in the ceiling that you could see by removing the bedroom roof vent assembly. If you do choose to put in a second a/c, I would recommend one of the smaller units and if you don't understand electricity, get some qualified help.
EDIT: I just looked at RV Trader and I found a 2021 Kodiak bhsl listed there for sale that has a second a/c and 30A service so it is pretty safe to assume that yours is prewired.
