...I'm going to look into an efficient, resonant antennas for 40-20 and whatever else is possible. But even that is a lot of wire...
It depends where you are operating and what you expect.
I've operated QRP over the years from many different locations.
Mostly I carried a dipole kit in my backpack that I could configure
for any combination of 80, 40, 20, 15 and 10m. (My rigs at the
time didn't cover the WARC bands.)
The only time I remember that I wanted to set up an antenna
and couldn't was while riding a train for a couple days across
Australia.
Granted, the dipoles weren't always set up in textbook manner,
and occasionally the wires were converted into other sorts of
antennas as circumstances permitted or required. And sometimes
it didn't work very well, like the wire tossed out the hotel window
in Alaska, with a coat hanger bent into a grappling hook to catch
the rain gutter across the alley.
But, at least for my style of camping, finding space wasn't a
problem in any of the campgrounds I remember.
And overall the antenna worked pretty well... but I often
used CW instead of SSB.
One of the cruelest statements ever made by a ham is, "just throw a wire into a tree." At 5 or 10w it had better be a very high-performing wire indeed.
An end-fed wire isn't as simple as an antenna as it might
appear at first glance. You need to consider the ground
system, often a tuner, and whether the radiation pattern
is optimum for the desired communications. Many of
the implementations are more suitable for running 100W,
where even a 10% efficient antenna radiates more power
than my dipole does at 5 watts. (Unfortunately, some
antennas advertised for QRP are more "cute" than efficient.)
That's not to say that an end-fed wire can't work well - it
certainly can. But it can also be pretty useless, and the
secret is understanding to know how best to optimize it
for your desired communications. I've used both types,
and that is why I ended up with the dipoles.