I've understood to use approx 15-20 degrees differential as the "target"
Also, prob a small difference in most cases, but measure the temperature of the air right at the cold air return..... the intake filter...rather than the temp at the counter. Several reason, but it could be colder there than on your counter.
I'd suggest doing this measurement is the very best first step you can make. It tells you if the unit is working...if it's not giving you that differential temp, then this points to dirty coils, low freon, electrical problems, etc...
I'd bet odds are really good that it's around 20 degree drop.... If you are, then it points elsewhere.
I'm finding that the unit on mine, Coleman Mach 15, is undersized for the need...it's probably not a very efficient/modern unit either.... all opinions shared by many on the Thor forum I follow for others of the same brand I have.
Anyway, I've found that if you get the RV really cold in the AM before the day heats up, then it might be able to struggle along through most of the afternoon before it heats up too much. Starting mid day to cool down a hot RV...forget it.
One fairly common problem it seems is the opening from the AC to the roof ducts is sometimes not opened up properly at the factory.
Some people have reported air leaks in the plenums that can be taped up to help a little bit.
I found just this past weekend that I've had a short circuit all these years...not electrical, but cold air coming form the discharge plenum straight over through a leak to the intake plenum, just recirculating...
another suggestion I've seen is to install another cold air vent right at the AC...
assuming yours has the roof ducting and a similar AC to mine....one half of the cover in the ceiling is filter. the other half is where the cold air comes out of the unit. Take that cover down and you'll understand. Well, some folks are buying one of those round vent louvers, i'm guessing 4 inch diameter or so, like you prob have in your ceiling vents.... and install that right in the middle of that cold air discharge side.... gives a straight path for the unit to dump cold air into the RV. I haven't done it...yet...but folks report that it helps.