What kind of mic would you use to record production sound in an enclosed trailer?
It's a relatively small metal trailer the size of a UHaul. It might be echo'ey so would a shotgun be less effective than a lav?
...or you could tell the directors and producers: "This is not going to sound good at all, just so you know, we might have to ADR or you could make it easier for me with the framing". Don't tear your heart out for an enviroment/film you know is going to sound like crap, even if it's a good script and it's good people. Good equipment can't save a bad sounding location...
But I think you should go for both lav and mic. So you can choose.
Wow...that sounds awful.
My first thought is how bad this'll be, and your approach, would be mostly driven by how the shots will be blocked and shot. If the mic could be positioned with its axis along the longer length of the trailer, I'd go hypercardioid, but if not, I'd go cardioid, and I'd bring enough sound blankets to cover the entire interior (even though, of course, you'd not want to cover what's in-frame). You'd need very specific reasons to go with a lav, IMO, such as if there is no room to boom from below given the shot width (the ceiling of the trailer will probably mean no overhead booms unless the subjects are seated).
Otherwise, what's your ADR budget? :-(
Blankets... and rent a nice hypercardoid. I looove the Schoeps CMC6 for very echoey situations. It's generally pretty cheap and totally worth it.
As Noise jockey says, blanket the place up as much as you can. I would say MKH50 for your mic but as you say there is no budget so try and deaden the sound as much as you can and use what mics you have. Maybe get some extra bodies in the trailer as long as they know what being quiet means.
And don't forget to record wildtracks at a closer distance or somewhere else.