what size cable for grounding my aerial.

Hi all I have a Major cb radio, aerial wise i have a thunderpole orbitor on my motorhome, can hear lots of foreign drivers but only one or two locals, I dont think any of them can hear me as i have asked umpteen times if i can be heard. nobody acknowledges im there, somebody said its because the aerial is fixed to the fibreglass body of the motorhome, so i need to know what size cable to run from the base of the aerial to the chassis.????????????

Welcome to the forum aswell.

thank you, someone who can hear me at last well almost.

5/0 here in Germany old man.

Are you off freq on the Major? Does it have a kc shift? Whats the SWR on the Thunderpole? Are the foreign drivers local to you when they are passing?

The Locals speak foreign language?

Ref your question on the size of cable, it depends on on high the Ae is fm the ground + 6ft under for decent earth!

the aerial is eight feet from the ground on top of the motorhome, the aerial is supposedly set so no need to swr it, what is kc shift i dont understand???

You sound like a complete beginner, ( as all of us have been,) & it sounds like you are being confused by the terminology. The earth that people are referring to is possibly better called a counterpoise. That is, what you need is the opposite half of the antenna you have mounted on top of your motor home, to make it work as it should.
With a vehicle mounted antennas this counterpoise is normally provided by the electrical contact with the vehicles metal bodywork. But if your vehicles bodywork is fiberglass then this is not possible without some sort of change to your current set up.
There are a number of ways of fitting antennas to fiberglass vehicles, but to use your current antenna that will mean fitting some sort of metal work, or fitting the antenna in a location that allows it to earth either with the chassis, or with a largish piece of metal.
There are antennas designed for vehicles such as yours, but to be honest, they are a last resort & there are sometimes other better alternatives available.
Is all of your vehicles bodywork fiberglass? do you have those metal steps fitted at the rear of your vehicle? if so that may provide sufficient metal work to give you a counterpoise, even if it is at a less than ideal position.
Without a counterpoise your antenna will not resonate at the required impedance & when you key the microphone this will result in a mismatch which will set up a Variable Standing Wave Ratio (SWR) of returning power to your transmitter & this will cause your transmitter to reduce power & if the SWR is high, which by the sounds of it, it is, then it may not transmit at all, as a result. Hence your problem.
The above is as simple as I can make it, whilst still I hope explaining what is happening & what you need to do, but is not a whole or totally encompassing reply, so please ask questions you want & bit by bit we can all help you get to a resolution.
Don't forget the only silly question is the one that you decided not to ask, that ends up costing you money, so ask away.