It's a feeling many of us experience, but thank you so much for looking and Wednesday isn't so far away !!!
Ta chuck.
Thanks for that, I do have some thick cables already installed by a member for a failed electrical update some time ago and when the job was re done the wiring left in situ but disconnectedIf/when you install the B2B you'll need to fit new thick cables between the cab battery (or better still the alternator) and leisure battery, the B2B basically goes inline with these cables. You could fit these cables ready for when a B2B becomes available at the right price and in the meantime simply fit a high current relay in the + lead and activate it with a D+ from the ignition switch ( you'll also need this D+ lead to activate the B2B)
This relay setup will give you much faster/ better charging than you currently have and you'll probably decide a B2B isn't needed. (You can only do this if you're using lead acid leisure batteries, NO Lithium!)
Trev'll be along in a while and post his schematic and link to a suitable relay. No doubt he'll suggest an optional switch to energised the relay so that you can assist the cab battery with the leisure batteries on cold mornings or the cab battery goes flat, It'll save me the job
Sounds like you're nearly there.
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35mm2 as a minimum is a bit OTTSounds like you're nearly there.
Cable CSA (guage ) and length will largely govern the charge current and obviously I don't know exactly what they are WRT your setup but 35mm min for shortish runs will give you some umph and ultimately anything will improve/add to the existing charge route.
Here's a relay below.
No need to faff with removing the existing split charge relay because you're basically just putting an identical but better relay/wiring in parallel with it.12V 200 Amp Heavy Duty Split Charge/Winch Relay for Car Van Boat 4 P KYURUK SN⤠| eBay
Model: 12V DC 200A. Current: 200A. The picture may not reflect the actual color of the item because of different photographing light, angle and display monitor. Size: 80 42 42mm/3.15" x 1.65" x 1.65".www.ebay.co.uk
Just a thought, all of the above assumes you've got a standard alternator, if it's a newfangled smart alternator then yes, you really need a B2B!
IF you need any help just ask
Agree completely David WRT wiring up a B2B for all the reasons you put and also because volts dropped over the input cables will be compensated by the B2B simply drawing extra current. However when trying to get a decent charge into leisure batteries directly from the alternator (which we were talking of) then ANY volts drop at all will significantly reduce charging capacity.35mm2 as a minimum is a bit OTT
The cable requirements will depend on a combination of cable run and current, but if say a 30A B2B on a run of maybe 3 Metres, there is no need to go above 16mm2.
If you look at the manufacturers installation guides for a typical 30A charger they will talk about 10mm2 and 16mm2 typically. Something like a Victron Orion-Tr Smart 30A you could not even get a larger cable than 16mm2 fitted in it.
Generally speaking, I am a proponent of going big on cables, but got to temper that against what is practical to run, practical to fit and also practical to buy (have you seen the price of cables recently? Say you went for a 70mm2 Copper (you mentioned 60mm2 but you don't get 60mm2 - goes from 50mm2 to 70mm2) and did a run of both +ve and -ve from battery to battery, you would be looking at over £30 per metre on the run. That is rather a lot of wonga when you don't need anywhere near that cable size (you might use 70mm2 Cable for a 2500W inverter pulling over 200A, but not a piddly little 30A or 50A B2B).
) but that carries power to the inverter which unfortunately I couldn't get adjacent to the batteries hence 70mm! Very conscious about the cost of cables hence the use of the chassis for the 0V/neg too!! 
uk.renogy.com
That's the one I've got David.FWIW, I have not tried one of these and have no intention of selling them before anyone says I am touting for business, but these look good value for money assuming they work as advertised ....
40A B2B currently on sale for £13012V 40A DC to DC On-Board Battery Charger
The 40A 12V DC to DC charger charges the 12v dc battery by 3-stage charging. The DC to DC battery charger prevents over-discharge and protect the generator.
Don't think there is anything on the market that offers that power for the same money.
Y know I've got that, I just didn't think, ta
Funney you say that as for the first time this week i had to do just that, think its time for a new starter battery as it over 10 years old.If/when you install the B2B you'll need to fit new thick cables between the cab battery (or better still the alternator) and leisure battery, the B2B basically goes inline with these cables. You could fit these cables ready for when a B2B becomes available at the right price and in the meantime simply fit a high current relay in the + lead and activate it with a D+ from the ignition switch ( you'll also need this D+ lead to activate the B2B)
This relay setup will give you much faster/ better charging than you currently have and you'll probably decide a B2B isn't needed. (You can only do this if you're using lead acid leisure batteries, NO Lithium!)
Trev'll be along in a while and post his schematic and link to a suitable relay. No doubt he'll suggest an optional switch to energised the relay so that you can assist the cab battery with the leisure batteries on cold mornings or the cab battery goes flat, It'll save me the job
