One way to warm up your bunk sleeping experience would be an Electro-warmth 12V mattress pad under your sleeping bag. Heat rises, so the mattress pad is more efficient than an electric blanket on top. It can run off your trailer's battery. Best to have a solar panel or something to recharge the battery, though, if using it for more than 2 nights in a row.
One reason why I like my Black Cat heater is, its radiant heat tends to be directional so I can point it toward the bed and get more effect if I need it.
