The best way to win proposals is to know the customer. All the best practices say so. Unfortunately, sometimes it makes sense to bid, even if you don't know the customer. When you find yourself in this circumstance, the best practices won't help.
The first thing you should do is to make a heroic effort to get the information you should have about the customer. We've written plenty about doing that elsewhere. But it's not enough to say "do the best you can" and then proceed like usual. You need to follow different bid strategies when you don't know the customer.
When you know the customer, the focus in your proposal should be on them and not on you:
How your offering will benefit them
How you will help them meet their goals
This is the best approach when you are trying to persuade someone that they should select you out of their own self-interest. But when you don't know the customer, you can't take the same approach. If you try, you risk getting it wrong and alienating the customer instead of persuading them.
When you don't know the customer, you have to base your proposal on the value of your offering. You can, and should, still talk about the benefits of your offering. But it has to be in the context of how your other customers benefit from it. You should load the proposal up with as many case studies and examples as you possibly can that demonstrate how others have benefited from your offering. Instead of talking directly to the person reading your proposal, you have to talk in terms of a typical customer. Examples are important to help the reader see how it could apply to them.
Even though you can't focus on the goals and mission of this particular customer, you can talk about how your offering has increased the capabilities of your other customers to fulfill their goals and missions. Even if you look up something on the Internet that says what the customer's mission is, you don't know the internal politics of the customer well enough to talk about them. So keep the focus on your offering and its value proposition.
To win you will need to turn your outsider status to your advantage. You can bring fresh insight, approaches, and technology to the customer. You can recognize the customer as the leader and enhance their flexibility and decision making capabilities. You bring options. While you are the expert in your offering, let them be the expert in their mission. Offer to enable and support them. You can't take or lead them into the future, but you can increase their capabilities, improve their efficiency, and help them make well-informed decisions.
If you are competing against someone who knows the customer, they will be saying how they are low risk, offer instant start-up without disruption, and can better support the customer because of the depth of their expertise. So make sure you demonstrate how you will mitigate the risks, prove that you can start quickly without disruption, and offer to hire any incumbent staffing to retain their customer knowledge. But give the customer a choice. The status-quo incumbent or your fresh capabilities. This and price are the primary reasons that people choose to go with someone new when selecting a vendor.
If they like and trust the incumbent, you will probably lose anyway. But if they are willing to stray, you need to give them a reason to select you. Your proposal has to give that reason to them. So make your proposal all about your value proposition, and make your value proposition add up to a compelling reason for them to want you.