A lot of people will cover their windows with clear plastic film leaving an air space of about 1 - 1½ inches. This gives a little insulation... That air space is too big; it allows air to circulate reducing the insulating effect of the plastic film.
I go a step further. Instead of using the plastic film, I use 1/4 inch "bubble wrap" - two layers with the bubble sides facing each other. This gives a lot better insulation than the plastic film and gives a degree of privacy. It lets light in but you can't see through it.
The skylight in the bathroom gets covered even though it is already double pane.
I also cover the screens in the screen door to eliminate the draft caused by the air current on the door.
Closing the vents on the A/C helps...
For heat, I use electric oil filled radiators with a small fan behind each one to circulate the warm air throughout the trailer. I have made a couple of humidifiers that mount on the radiators to keep the air from getting too dry. The furnace is set to 55° and it only comes on if there is a shore power failure.
Last winter I was able to maintain an almost constant temperature inside the trailer of 73° - 75° when the outside temperature was below +10°. It did get down to 68° inside one night but the outside temp was -15°.