Robin, smoke testing, actually "smoke tracing" is a means used to visualize patterns of air movement. I've used it to verify the integrity of containments used for hazardous materials abatement, demonstrate air delivery and exhaust patterns etc.
You would need a place with no wind like inside a garage.
You will need an air mover, a shop vac will suffice.
With the shop vac outside the trailer, plumb the EXHAUST from the vac into the trailer. This connection should be as air tight as possible. One method is to cut a piece of corrugated cardboard to fit an open window. Cut a hole in the cardboard to fit the vac hose through. Duct tape hose through cardboard. Duct tape cardboard to open window.
Close all windows
, doors, vents etc.
Activate vac. Use your smoke tube to puff smoke around windows
, vents, corners etc. where there may be air leaks
Have someone outside the unit watch for smoke exiting your unit.
Conversely, with the suction from the vac you can create a partial negative pressure inside your unit and have someone puff smoke around potential leak areas from the outside while you sit inside and watch for smoke infiltration.