Connecting a Generator to easy solar 2 48/5000

Hi, we are installing an easy solar 2. we have 5 US5000 batteries connected via a lynx device. The system is off grid and we need to have a function by which when the batteries hit a crucial level someone gets alerted and goes to site with a generator. They would then charge the batteries form the generator. Reason for this is the site has lots of problems with diesel theft so we don’t want to keep the generator on site. I believe we should use the AC in for the generator but i wanted to check an ATS isn’t required as we will be manually charging the batteries.