Bongo Fury

I have a split charging relay and I can see that 13V is being delivered to the leisure battery. Is there any way of telling the amount of charge going to the leisure battery? It went flat a few weeks ago so I'm wondering how much charge it is getting while I'm driving.

Also, what is the best gauge or other method for monitoring the charge state of the batteries?

The only way to see how much charge is going to the battery is to use an Amp meter in line, or use a DC Clamp meter to measure it. You can expect 20+Amps initially if the LB is flat this will then reduce down over time to about 1-5 Amps as the battery charges.
As to gauges, there are lots of simple LED 3stage lights available, but to be honest measuring the voltage only gives an indication and a very crude one, but better than nothing, the voltage really is only meaningful when the battery has been at rest for about 30 mins or so with no charging or load on it. At this time of year unless your doing long trips there is a good chance you will find both SB and LB going flat if your not carefull! My rule of thumb is that you need 20 minutes driving on dual carriageways to replace the the energy used to start from the SB, double that maybe if you are charging a LB as well.
There are more accurate ways but the cost gets significant

http://www.nasamarine.com/proddetail.ph ... BM1Compact

Something like this would do the job but is perhaps overkill for a Bongo