I have a rig with a very similar floor plan (Palomino Puma 25RS, no slides), except there is a sliding door separating the front queen bed from the rest of the TT, and the bathroom is near the bedroom, providing a big rear window.
With my experience with an electric heater, I'd go for the 5000 BTU/hour plat cat model at minimum, especially the few hours before dawn where it gets quite chilly. A 5000 BTU/hour heater will be able to handle low 30s/upper 20s, in my experience. However, anything more than that might be iffish. Your milage may vary -- TTs vary in insulation, and what might be enough heat for one person with a four season trailer will be woefully inadequate for someone with something with less insulation.
You are wise to choose a vented heater.