I had to research this because I had the same problem as Rod.F - breakaway battery flat but not charging from the SETEC power supply used to charge the main caravan battery. In normal circumstances, the breakaway battery should trickle charge via your trailer plug while you're driving to keep it topped up. However, if the van is stored for several months, it will eventually go flat even if you keep 230V connected to charge the main van battery. In this case, you can buy the RV Electronics charger or buy a simple (single stage) 15V plug pack battery charger (used to charge portable booster batteries) from your local auto accessories supplier.
Knowing a bit about batteries, I assume the reason why the breakaway battery must be charged separately is that the breakaway unit only uses a simple trickle charge circuit (e.g. a resistor in series with the battery) which, if left permanently connected, would sulphate the battery and eventually destroy it. Thus, while the SETEC II unit may be left switched on permanently to keep the van battery fully charged because it is a sophisticated multi-stage charger, the breakaway battery charger should only be left on for a day or so and then switched off because the battery should be fully charged by then. A battery in good condition should only need a topping charge every 3 months or so.
The SETEC II unit actually automatically disconnects the charging voltage once it detects that the battery is fully charged and restarts automatically when the battery terminal voltage falls below a predetermined value (13.2V I believe). Now, if the breakaway battery was also connected to the SETEC, there would not be enough voltage (at 13.2V) to trickle charge it fully via a resistor and you would otherwise have a large capacity (100ah) battery and a small capacity (7ah) battery, each with slightly different chemistry and very different electrical characteristics, connected to a charger designed to charge one battery or two similar batteries.
Hope this helps.