Assume nothing
But yes I would assume that the 2 under the seats are leisure batterys which I would expect should be wired in parralel to give you 2x the capacity. But then thats making assumptions. Perhaps they are seperate and you need to switch them over in some way..
TBH the only way to sort this is to so as people say on this site and take your time doing some diagnostics.
First thing buy yourself an electrical meter for about £10 from maplins. test each of the 3 batterys to see what the voltage output is from each. I suspect you might have a knackered leisure
battery that is drawing power away from the other. If you do need to replace one it should be the same size as the other. ie you need to have 2x110 not 1x110 and 1x80 amps
If thats not the problemo
Then I would disconnect each
battery in turn and check when your van meter looses its reading and also when your sat system stops working.
Then I would reconnect the engine
battery, see what if anyhting has current, should only be the engine but if you have a switching system perhaps you can run your leisure circuit from the engine
battery. We can do this with ours but of course never do but its nice to know in an emergency we could. As said some self seaking satalites are wired into the engine circuit as they have to retract automatically when the engine starts
Next I would try to work out which of the 2 leisure batterys is the master ie which is connected directly to the leisure circuit, reconnect this. See what has power and check the voltage at the terminals. Then start the engine, when you check the voltage at the terminals again it should be higher showing that you are getting a charge from the engine.
Next connect the other
battery and do likewise. Again with the engine running if its been connected correctly you should get a higher reading at the terminals. if not then perhaps there is a wiring fault.
Good luck