Being a 12VDC fan motor you should be able to make the fan motor rotate in the other direction and allow it to blow into the trailer... Just reverse the 12VDC polarity with a DPDT switch from Radio Shack.
We have the fantastic fan over the bed and we use ours mostly in the exhaust mode where it pulls cool shaded air through a couple of open windows in the other part of the trailer. Love our fan.... I never have looked-up how many amps it pulls from the battery but is on probably 24/7 in our off-road POPUP camper when camping off the power grid.
I can unzip the window flaps on both sides of our off-road POPUP just a few inches where rain doesn't get inside and it sits this way for the whole trip.
Roy Ken