22 feet long? Then 13,500 Btu's will cool that space really well.
However if you have canvas ends, it might not do so well. If you could describe the RV a bit better, it might help. Model year as well.
Do you plan on running from a generator a lot?
Did the old one fail due to low voltage? Just wondering. Many do not check the voltage, and many 30 amp campgrounds tend to run less than 105 volts on a hot day. . .
Some in small campers want to buy a smaller unit, like 8,000 Btu, so that the compressor runs a longer time before cooling the 8' wide X 12' long camper off. This helps with humidity, something you do not need to deal with in Washington. You just think it is humid up there. Also the smaller units use less power, save gas in the generator.
Normally something over 30' long might require a larger 15,000 Btu unit, or perhaps dual units over 34' long.
Good luck,
Fred.