We had the Portable tanks with connecting collars. The could gravity feed other tanks. as long as you had 2 collars on each tank there is no limit to how many tanks you can connect except for flat ground area. In one industrial Fire we had 3 ladders and 8 2 1/2" handlines going constantly for 3 hours. From 4 fill stations.
You should get some department to showcase your drain sleeve connectors. Connecting as many portable tanks as possible. The Village of Salem Lakes, use to be known as Salem Township Wisconsin Fire and Rescue use to use those and had 8 3500 or 4000 gallon tanks. I had been on that department when we had all of our tanks plus those of Neighboring departments connected up. We actually had wished we had 3 or 4 drain sleeves on those tanks so we could have used less hard suction to connect additional tanks.
Certification iso pour le volume d'eau qu'ils peuvent pomper sans arrête pendant un nombre de de temps donnée. C'est pas tout le service qui utilise des bornes fontaines.