That's absolutely true. There are three main ways the dealer can make money on a deal. They are; New car price, Trade in value, and Financing terms. As a consumer you should always negotiate each of those items separately. What dealers like to do is move their profit between the three to make it look like they are negotiating when all they are doing is moving the bottom line costs from one line item to another.
So for example they might quote you a price on the car that they know is below average asking price from dealers in the area and give you a typical good interest rate on the loan but then way low ball you on your trade-in.
To avoid getting a bad deal I always do the following before setting foot on the dealers lot.
1. Go to CarMax or similar place ahead of time and get a quote for how much they would buy your car for.
2. Be ready to pay for the car in cash or have a loan pre-approval lined up from a secondary source for financing outside of the dealership.
3. Know what the car you are looking at sells for in your area.
4. If you are buying a new car, shop towards the end of the month. Manufacturers offer incentives to dealers based on the number of cars they sell per month. So say a dealer needs to sell 50 cars a month to get the incentives which are $500 bucks per car. If they are at 48 cars sold with two days left to go in the month they are a lot more likely to sell you a car at a lower price and get to 50 so they don't lose out on the incentive payments for the 48 cars they have already sold.
Once you are at the dealer make sure you negotiate each of the 3 parts of the deal separately. First agree on the price of the car. Then entertain their offer of a loan and for your trade-in if they are as good as or better than the alternatives take them other wise turn them down.