IMO, you need a generator (or two generators paralleled) that produces at least 2800 continuous watts (not surge or peak watts) of power in order to power a 13,500 BTU RV a/c with a very high success rate. In order to do this reliably and consistently (especially with a 2800-3000 watt generator), all other 110 volt appliances in your RV, including your converter, should be turned off.
Many have good luck powering their 13,500 BTU a/c's with the Yamaha EF2400ISHC generator (2400 watts continuous) which is an outstanding generator with outstanding sound level specs. However, there is a small minority that cannot. For those folks, they have to resort to changing capacitors in their a/c unit or getting another generator. For some, this is too big of risk when you're spending $1400 (street price).
In order to make sense of fuel consumption rates you have to know time, amount of fuel consumed, and load. Unfortunately, many manufacturers are vague about loads. Usually a call to the manufacturer will get you this information.