The heat strip on my Carrier LP 13.5K unit is 1500
watts and does a good job keeping the trailer warm. I ran it with the outside temperature in the high 20s and it kept the trailer around 70 degrees.
I replaced the AC on my 75 last year and it was held on with around 50 screws. After removing all the screws and the AC, the holes were filled with Olympic Rivets and some Vulcum. You then need to install braces between the inner skin and the outer skin so that the new unit can clamp onto the shell. Forums member Zeppelinium made the braces which we installed.
Getting the old unit off was much harder then getting the new unit on.