Answer: By your question, it appears that your community is not sub-metered. If it were, the owners of the home would likely recognize the problem and immediately and fix it.
In my experience when water is included as a part of the base rent, most owners really don't care, and don't check. But when the community institutes a sub-metering program, everyone becomes an overnight conservationist. Sub-metering is a win-win for everyone; the landlord saves money in not having to pay for wasted water, and the residents save in (a) controlling their own water bills, and (b) not having to suffer needless rent increases to recapture the cost of wasted water.
Now to your questions. Clearly, if water is visibly running out of the home, the tenants should be notified and told to fix the problem. They are responsible for their own homes.
As for the less obvious leaking problems, the only way to find out is to survey the tenants on the issue; e.g. do they hear the toilet leaking, for example.Same question for faucets. Next, what about under the home? Has anyone checked lately? I have heard of management offering to do inspections under the home for free, as a part of instituting a submetering program.
But can you require residents take these proactive steps, especially hiring someone to inspect under the home. Except for the rules regarding the siting of home on a space, there are likely no regulations that mandate such action on an ongoing basis - at least if there is no present evidence of leaking. If there is evidence, ORS 90.740 can be relied upon to secure compliance, if nothing can be found in the rules or rental agreement:
90.740 Tenant obligations. A tenant shall:
(4)Except as provided by the rental agreement:
(a)Use the rented space and the facility common areas in a reasonable manner considering the purposes for which they were designed and intended;
(e)Install and maintain storm water drains on the roof of the dwelling or home and connect the drains to the drainage system, if any;
(f)Use electrical, water, storm water drainage and sewage disposal systems in a reasonable manner and maintain the connections to those systems;
If the rules do not contain such a provision, consider amending them to add language to address the issue. Rule changes can be done in a fairly straightforward manner. See, ORS 90.610. Alternatively, even if submetering is not addressed in your rules, you can unilaterally add it to your rental agreements, as a "Plan B", if you are unsuccessful in implementing the necessary rules.