Do we put it in our proposal as a free option for the first 90 days, then start billing?
I understand your logic/approach, but I think you have it exactly backwards if your goal is to increase RMR and really push this service.
I would make this a standard line-item in your quotes. Describe it succinctly and accurately "Network monitoring with proactive outage notification", or something like that. Have your pitch down solid in terms of benefits to the customer, how it can help them save on downtime, service calls, lost video, potentially detect cyber breaches, etc.
In the terms, tell them they can cancel at any time in the first 90 days and get a pro-rated refund, after that it renews annually. You need to play on the basic "momentum" of RMR billing, where once it is initiated, customers are less likely to cancel if the service/offering has perceived value and is not a "considered purchase" (e.g. so expensive that they have to stop and think about how else they could use that money).
To really make this stick I would probably look at subscribing to some services and news feeds for network and data center outages in your area and send customers proactive emails when there are general outages. This would be an email along the lines of "We are monitoring your service, and while your systems are all up and functioning, we wanted to make you aware of XXXXXXXXXXXX, which is currently affecting some customers in our area. If you or your employees are having trouble accessing XXXXXXXX, be aware this is most likely not the result of problems with your equipment or internet connection, but we wanted to make you aware of the issue as part of the monitoring services we provide you. Blah blah blah...."
Your challenge is that you are selling insurance, and if they do not use it, the perceived value may drop. If you provide some notifications of general outages or other issues it can help show value for the service and show that you are proactive and informed. If your customers have constant outages due to their own bad luck, or other issues, then you might not need to provide any additional updates, they will get enough value just from the direct monitoring.
Some examples of things you might want to check out for monitoring internet outages:
Downdetector
AWS Status
Spiceworks Outage Heatmap
In short, bill them from day 1, provide clear value/feedback from day 2 (or as close to it as possible).