There really isn't any hard-n-fast rule about a G5RV's height, other than saying higher is better. If it's high enough so that the ladder line portion isn't close to touching dirt, it's usually high 'enough' to work. Not well, but 'work'. A G5RV is a compromise antenna at best, and was only intended for use on 20 meters. It's marginally useful on harmonic frequencies, but so are other antenna. I guess you can tell I'm not exactly thrilled with G5RV's.
If you don't have room for a real live 160 meter dipole, then use whatever room you have to get up a 'shortened' version of some kind of 160 meter antenna. They take some effort, but an inverted 'L' with a very good ground radial system works very well. Again, it's a matter of as much vertical height as you can get and then the rest horizontal. A loaded antenna of some kind will 'work'. Any loaded antenna isn't going to be as 'broadbanded' as an unloaded one, but, if it fit's the room you have, it's better than nothing. For most of us, that 'better than nothing' antenna is about all that we have room for, especially on 160 meters.
A multiband antenna to cover all bands is certainly possible. Sometimes isn't to practical as to price, or abilities, but certainly possible. Just depends on how much you are willing to put up with or do without.
I'm not a fan of vertical antennas in general. There have been one or two that I thought really did well (on both ends of it), but they are not cheap or simple, and do require a very good ground system. One is a 'HT18' ('18HT'?) 'HighTower', the other one is basically the same thing except you build it. The hardest part as far as I'm concerned is that ground system, I hate messing with them.
One of these days... as soon as I win the lottery... yeah, right.
- 'Doc
