The ladder line portion of the G5RV is the matching network.

You cannot use a tuner with a tuner.

If the matching network is the ladder line and you connect a tuner to it - yes you can trick the transceiver into believing that is is seeing a 50 ohm matched load - but all you are going to create is heat.

On the other side of the coin, I hear all the time - I can work everything that I can hear - with my G5RV - the problem is - what can you hear?

Unless you have a real 80 meter dipole and you compare them side by side - within one hour of each other, at the same height and in the same neighborhood - you cannot compare the two.

In the end - you will realize that the efficiency is so low - you are not hearing much - just the strongest of signals - when the band is open, and not much of anything when the bands are no cooperating.

The thing that tricks people into thinking that they are doing something is the fact that they see 100 watts into the meter and they think that they are modulating all 100 watts - when in fact a single side splatter signal is only fully modulated part of the time - most of the time - we aren't really using more then maybe 15 or 20 watts out of 100.

Only the digital modes and CW - which is the original digital modes - dots and dah's - is 100% fully modulated.

That is the reason why we turn down the power when we work digital modes.
Most transceivers do not have a 100% duty cycle - hence if you operate at 100 watts for very long - your transceiver will not take it!