Trickle charger is relative...
The current flowing into the battery is dependent on the state of charge in the battery, the size of the battery, the output capability of the charger and the charging voltage.
Trickle charge is better defined as "float" charging.
Here, the curent should be about 1% of the battery amp hour rating.
So if you have 4 each 100 amp hour batteries in parallel, then 1% of 100 is 1 amp, x 4 is 4 amps, so the charger needs to be able to output constant 4 amp minimum current to satisfy the needs of a normally charged battery to properly float it.
A 1.5 amp unit simply will not do.
Next we need to be sure the voltage does not exceed what the battery is happy with to avoid gassing.
A 2-4-8 batteryminder would take care of your needs.
Tony & Lori
1989 Country Coach Savannah SE