The two things that will improve your signal the most are:
1) getting the antenna out of the metal trailer, and
2) height above ground
Antenna gain by itself comes a distant third, once you get
past a rubber duck to a full-sized antenna like a J-pole,
ground plane or 1/2 wave dipole (basically all the same thing).
Can you hit a repeater 20 miles away at 5 watts? Depends
on the terrain between you. I've covered 100 miles with a 2m
HT on a couple occasions, but usually that requires one or
both stations to have some height above the ground in the
middle. If you have a hill between you and the repeater it
will be more difficult.
Raising your antenna from 5' to 20' is like increasing your
power 10 times (on 2m over flat terrain to a station 10 miles
away). That's without consider the improvement from
switching the rubber duck to a full-sized antenna. Antennas
like a
ground plane are not difficult to build, and have enough
bandwidth that a minor error in construction probably won't
make the antenna unusable.
And often you really won't know how much power and/or
antenna you need to hit a repeater reliably until you actually
try it, especially when there are some obstructions. That's
another advantage of building simple, cheap antennas: you
can keep trying different versions until you find one that
works, then build a more permanent version, rather than
spending a lot of money on each antenna that doesn't work.
Oh, and do check that you have the repeater offset and tone
properly programmed - that certainly can prevent you from
hitting a repeater regardless of how close it is to you. When
in doubt, drive close to the repeater and make sure you can
hit it there.