Leisure batteries testing/charging
Take them to a retailer that sells batteries, preferably an RV or marine place that deals with deep-cycle types, but an auto parts retailer can test them too. They can tell you their condition - have they dried out, sulfated, etc. and whether they will accept a charge.

Trojan themselves have more info here, including how to inspect and test them: https://www.trojanbattery.com/tech-support/battery-maintenance/

And general info about charging lead-acid: https://batteryuniversity.com/learn/article/charging_the_lead_acid_battery

There's nothing special about these batteries as far as charging. They can be charged with a normal lead-acid charger. It will take longer for the big batteries, but they will charge eventually if their condition is still serviceable.

There is a method to 'restore' sulfated batteries using a pulsed high-current charge. Whether it works or not is an open question.