Regarding the pushing/pulling of things up hills. A quick scribble reveals that given an incline of A degrees, and pushing/pulling a mass M up this hill against gravity, this requires the same force as lifting (vertically) a mass Meff, given by

Meff=0.017 M A

for small (i.e all realistic) inclines A. To use the previous example, pushing the 3000 lb car up a 1 degree incline is like lifting a mass of 51lbs, which is perfectly doable. A 500lb trailer would be a mere 8.5 lb equivalent. Obviously bicycle gearing also becomes the issue.

For human propulsion, comparing the trailer mass to the human's mass (i.e. the ratio) is relevant, since humans are generally configured to be able to shift weights comparable to their body weight.