Solar panel extension lead

What sort/type of extension lead are we talking about ?

I'm no electro-tech, but I can't see why not. I have done the very same for my panel. Several things that I recall from my readings...1. make the extension cable as short as you can (minimise resistance). 2. Use decent gauge* (more wires) cable for the lead (again, minimise resistance). 3. The supplied regulator will work better IF it is close to the battery, not at the panel end of the lead. As a result, I removed my regulator from the panel and rewired it to the end of the lead. With a few metres of lead, one can position the panel for best sunlight when camping.
*I don't know the gauges lingo...I just had some 'meaty cable' in the shed :-).

If you are talking about how long can a lead be from the panel and regulator to the battery then it all depends.

You should use cable specific to solar panels and with anderson plugs for the best high voltage connection and quite thick cable. Good quality cable is around $10 per meter and we run 9m of it. Connections must be tinned and soldered never crimped

Rob

Utopia:
Humongous solar panel, liquid cooled, stuck onto regulator glued to side of battery (also liquid cooled). All wiring 1m in diameter and no longer than 1mm.
Reality:
Solar panel in the sun (getting hot), wire (variable length), regulator near battery, battery in shade so fridge can be near battery also in the shade.

I use anderson powerpole connectors to connect all of my solar equipment. I have several 'extension cords' with powerpole connectors on them of varying length (up 4-5m) and park my vehicle in the shade and use what ever length of cable I need to keep the panels in the sun. Wire gauge used is whatever I can stuff into the powerpole connector.
Do I lose power in the extension cords and connectors? Of course I do. Do I get enough charge into my system by keeping the panels in the sun and do I save more power by keeping my fridge in the shade? You betcha.
Is someone else's system better than mine? most likely.
I live in the real world and drink cold beer. ymmv

What's a reasonable gauge for say 5 metres with 18v going down it?

IF its a portable panel then you'll need an extension lead, if possible it is better to have the regulator at the battery rather than the solar panel end as the voltage drop to the regulator will not have as much impact on charging as the voltage drop being between the regulator and the battery.