Short answer, don’t drill. Trillium
is single wall which means putting holes through the trailer. In addition, walls are flimsy. Original upper bunk was attached to the wood frame of the window, the bottom horizontal piece. First you need to remove windows
and inspect wood framing. That bottom piece is prone to rot. Attaching bunk to rotten wood is not good.
Replace any rotten wood, reseal opening and reinstall window. Lots of prior discussions on that topic. My hand is full of some of the rotten wood in that lower front window framing. Obviously not going to support an upper bunk in that condition.
Many of the lower front benches have side doors opening into the aisleway in the trailer. Mine does.
No such thing as a 12 foot Trillium
, it’s either a 1300 or a 4500. (13 or 15 foot). There were other models of Trilliums too.
If you have a fiberglass interior, pretty much all of it is structural. Remove at your peril. Adding side doors, properly placed, should not be a problem.