This is what works for me
The easiest 'rule of thumb' to me is watts in a service and how I explained it to my non technical DW
30 amps = 3600 watts @120VAC (36 100 watt bulbs)
50 amps
per leg is 12,000 watts(120 100 watt bulbs)
Since I carry both 30 and 50 amp cords and adapters make my decision on expected outside temperatures and whether I need both air conditioners. I also factor in length of stay when traveling. Overnight, normally the much easier to handle 30 amp cords, And the DW, she has been 'trained' to not operate too many ' hogs' at the same time.
What
'they' don't tell you is that a 50 amp RV service is really 100 amps and that each leg or conductor is capable of carrying 50 amps.
Easy formulas
20x120 = 2400 watts
30x120 - 3600 watts
50x2x120 = 12000 watts
If the voltage varies, so will the wattage. If a CG system is less or more, then your wattage will change accordingly