The ampage via a standard alternator (more on this later) only stays high for a very short while, it only stays high for under a minute, rapidly falling as the terminal voltage rises and the internal battery resistance drops.
The main reason the fuses do blow is on heavily discharged house batteries when the relay cuts in massive ampage can flow (dont forget that even if the alternator is only 65 Amps say the Starter battery is also in circuit and this can easily supply 200 Amps (think starting !!) again unless the house battery is really wrecked it will be very brief, if the fuse as blown it means the leisure is either unservicable or as been really over drained
Alternators are power supplies and not chargers, the starter battery only has to start the vehicle, after that it is a capacitor only and does not supply the power for running the vehicle, A starter battery rarely if ever gets charged to more than 70% (unless you bench charge it), but as long as it fullfills its primary task which is starting the vehicle no problem.
Why only 70%? well the standard voltage regulator reacts to terminal voltage, its set to keep the voltage around 13.8 to 14.2 Volts, this is not enough to keep the ampage flowing into the battery, hence it will never fully charge, what happens is that while the regulator is reacting the battery smooths, the regulator catches up the drops back.
They could set the voltage higher, but it starts the vehicle so why change? If you look into the charging regime needed for leisure batteries even the weakling Gell and AGM Types want a minimum 14.5 Volts and a decent lead acid wants 15v, if an alternator was designed to charge it would be set higher. The standard regulator is a compromise which works perfectly well for the starter battery but is pretty useless for charging a house battery to a decent useable level.
The only problem for us is that we want to get as much use as possible from our leisure (house) battery, to fully charge we need something that stage charges to a decent level, hence a smart regulator this takes control of the alternator keeps the voltage high and keeps the charging at right level until the battery is fully charged.
This means a 100 ah house battery will be good for 50 ah use, rather than 20 ah from standard alternator charge, a side benefit is that the cold starting ability of a fully charged starter is way better than the 70% achieved previously.