stainless bulk milk tanks -how to daisy chain from top

[YaksSap1]

I have 4 stainless double wall milk tanks that I want to cascade as my electric releaser pumps up to them. Meaning I would like to flow sap to them so that they fill in sequence. I have them lined up with elevations dropping to lowest tank. Given they are double wall (insulated) I am seeking a way I can have an overflow pipe from on to the next. I know I can tie all the drains together, but am concerned with freezing at drains (1.5 inch). Anyone done this alternatively successfully?

I could set up a tote with bulkhead fittings to fill and then utilize bulkhead fittings to "octopus" to all four tanks but would like to try filling the in sequence instead.

Thanks for any ideas or experience you have tried.
Yaks Sap

[arcticmaple8]

We had 3 small flat top hooked togeter at bottom. It was a pain so we sold them and got enough money for 1 bigger used h2o sap tank.
To hook them together u could drill or cut a big hole through outside jacket and put bulkhead in tank in interwall. You could weld a pipe in also instead of bulkhead fitting if you or someone you know has a tig.