Pete,
KTManiac is correct.
http://www.mrheater.com/upload/78438...B_BL_rev_C.pdf
The Big Buddy user manual refers to the ventilation requirements on page 3. It describes 18 square inches as a 4 1/4" x 4 1/4" opening. In our rigs that is pretty easy to do. Some times it seems like it's just as windy inside as outside. Anybody that has gotten up on a cold windy night and stood by the front door in his boxers can attest to that.
Bottom line is we need to keep our loved ones safe. Read, understand, and follow your manual instructions and you should be fine.
We run at 18,000 BTU just to get the Monty quickly warmed up. Then we reduce the heat to medium or low depending on the outside temps. Last month at Tonopah, NV it was 20 degrees outside and we were a toasty 70 degrees inside running on medium. Hope you enjoy your new mod. Keep us posted on it's performance.
John