Exterior caulk?

vertical or horizontal surface?

Vertical

If you are looking at the SIDE seams and holes, you can't go wrong with sikaflex 221 as it is built for RV'ers and will withstand temp changes, constant motion of rving etc. Can get it at most rv shops or camping world for around 9-12 dolalrs a tube. If using on the top you need to match the sealant with the type of material you have for a roof, and choose self-leveling HOWEVER, many are changing to Eternabond tape as it seals better and appears to last longer and can be applied to almost any roof surface.

Vertical is Dicor NON LEVELING

Dicor lap sealant or Proflex rv sealant for sides and Dicor self leveling for roof seams and vents, etc.
Do not use anything with silicone.

acritzer wrote:
What's the best caulk to use on the exterior of the trailer? After close inspection I'm missing some near the top on my side wall.

From the gutter rail down use Pro-Flex by Geocel...

Sounds like the surface was not cleaned well if the caulk is missing. I would start with removing all the loose caulk and cleaning the surface good with acetone then use Geocel as posted above it has more of an adhesive property the the Dicor or other caulks that I have tried. It dries firmer and does not crack.

My favorite is 3M 5200 for permanent installations and 3M 4200 for general purpose. Both are non-silicon, retain excellent adhesion and elastomeric qualities, AND are paintable AND are designed to hold their qualities while underwater.
I never use self-leveling anything.
My second choice is Silaflex caulk/sealant that is non-silicon.