"If your site was cut and the tank was on the boundary, then you would have some cause for concern due to the tank's zone of influence."
When we rebuild, the land was cut and filled. Do you mean this or do you mean if my land was lower than his??
From what I understand from your mention of weep holes, the sites are now level. If the tank was on the edge of a cut boundary and there was no wall, the situation would potentially be dangerous due to the tank's zone of influence, the reason why water tanks should never be installed close to a cut boundary. Even when a retaining wall is installed, a water tank's additional zone of influence may exceed the retaining wall's engineered parameters.
Does the poly slimline tank have flat wall sections? That type are the most vulnerable if there is a manufacturing/design shortfall.