I have two 16 plate Vivaro's. Both have a twin setup with two pumps, two controllers & one 110 leisure battery. Both vans are mostly used by just one man all week and we almost always work very local. Controllers (Spring) are set to around half power (40-60).
We have a Sterling battery to battery charger in each van and they have never given us problems in the 3 years or so we've had one system & a year the other. I use a Ctek mx5 most weekends to fully recharge the battery in each van. I bought an extra set of ring terminal fly leads so as neither battery has to leave the van whilst being 'bench' charged.
My point is that although the maths may not add up, in my real world experience the Sterling battery to battery charger keeps the leisure battery topped up enough in each vehicle to not give any problems. Some weeks I don't 'bench' charge either, especiallly if we've had a quiet week. I think I could possibly get away with not bench charging at all.