You can't go wrong with a 9000 series Polar Cub in a TC. A 7000 to 9200 btu works great. I have the 9200 btu unit with heat strip. I would not go larger than a 1100 btu unit. The 9000 series Polar Cub works well with one Honda 2000, and with mine I can still charge my battery.
It is my understanding from reading the Forum over the years, if the A/C is too big for the RV, it will not be as efficient. It will recycle more often and for that reason the on/off can be very annoying.
I really question the value to the heat strip. The wind chill seems like it overrides the heat you gain. When I bought my Polar Cub 9200 btu unit (new-2005), the heat strip was an additional $50. To add it later was $150.
If you buy a used A/C look close at the outside cover. Over time, they become very brittle, and a simple brush against them can crack the cover. It has been a few years, but the last time I checked they cost over $150.