Are you planning on living on the canal side (constantly cruising) or in a marina? If this is your first boat then a marina for a few months might be worthwhile while you get the hang of power supply etc.
If in a marina, simply plug in the landline into the bollard and the Victron will look after your batteries.
If you are constantly cruising then running the engine regularly will also charge the batteries directly fro the alternator(s), but insufficiently unless you are cruising 5+ hours a day. Additional charging can be by simply running the engine whilst tied up but charging this way will never get the bateries back up to 100% which is what you are aiming to do. Attaching a generator to the Victron will make a much better fist of charging to 100% but you really do need a good battery monitoring system to keep you informed of the state of charge, Everyone has a view as to the best Battery Monitoring System but hopefully I won't start a riot by saying that a "Smartgauge" (reasonably priced and easy to install) will give you a good idea of when the batteries need charging and a monitor to tell you the charging current and therefore when the batteries are approaching 100%, as described by Alan de Enfield, post 3, shouid suffice.
Petrol generators have to be treated with respect. Apart from sizing, as descibed by BEngo in post 2, they are potentially very antisocial if not properly silenced and dangerous, don't even think of putting one in the engine room and remember that all engines, if not being used for propulsion, are banned between 8pm and 8am.
Sorry crossed with AdE's reply, same message different words! I'll butt out!