You need to check (1) local zoning codes, which may well specify one tank/trailer or a certain amount of tank capacity per trailer; (2) local health and sanitation codes, since if septic refuse or products are escapting, that coudl be a health violation; and also (3) any leases or other agreements, to say what they say about  the subject. A lawyer could help you do this, or if you don't want or can't afford an attorney at this stage, go to your town hall or municipal office building and speak with the heads of the building and health departments (or the town clerk, if there aren't separate heads for those departments). You should be able to determine fairly quickly if there's at least a reasonable chance of a violation; if there is, you can either see if the relevant agency will act or else get an attorney to help you take legal action.