How long it takes to charge leisure batteries while driving depends on the alternator’s output current and the size, spec and discharge level of the leisure battery bank. To be conservative in our calculation, we assume the alternator is only 50% efficient. So a 140A alternator will only provide 70A charge per hour. Now take a look at the battery bank.  Let’s say it’s 200ah made up of 2 x 100ah batteries each with a maximum charge current of 30A and it’s 70% charged. We need 30% of 200ah (60A) to fully recharge the battery bank. The batteries can accept 30A each per hour so 60A in total. The alternator provides 70A so the battery bank will take a little less than an hour to fully recharge.  This of course assumes the campervan batteries have no load. They’ll charge quicker if you have solar panels too (and it’s daytime). Please note that in reality, split charging can only charge a battery to 80-85% of it's total capacity.