Can only speak for Ford dealers in this area.
You can get a new Superduty for invoice price minus any manufacturer rebates.
If you go to nada.com or kbb.com or similar site you can see the MSRP price and the invoice price for the base truck and options. From that you can calculate what the % off the invoice price is. That % will vary a little based on specific vehicle and options. But you can use that as a rough estimate of the invoice price when perusing the trucks on the lot. A quick calculation will give you the approximate invoice price for any truck there. In round numbers, if the calculated % was 10% and the MSRP of a given truck was $50,000, then the invoice price would be approximately $45,000.
From the same web sites, you can find what current rebates the manufacturer is offering. That rebate goes to the customer so you can plan on the invoice price minus the current rebate. Basic Ford Superduty rebate right now is $2,500. There are also quite a number of smaller rebates available you might qualify for such as Ford Credit financing, military, and many others. At one time they had extra $500 off as a small business owner.
Sometimes the manufacturer offers the dealer extra cash incentives to move certain vehicles. That goes to the dealer but if you know he is getting it you might be able to bargain him out of part of it if he wants to move some stock.
Ford pays the dealer 3% holdback on the MSRP price of any vehicle. So for every $10,000 in MSRP price he receives $300. Plus he may receive other incentives based on sales. So even if he sells at invoice he is still pocketing some money. But that is not all profit as he has to cover his overhead out of that also. If he sold everything at invoice he probably could not stay open. But most of his sales will be well above invoice.
Check local dealer web sites to see who is offering the best special offers/rebates. You can also check each dealer's inventory to find just the right truck. Then you can use the best offer of one dealer to bargain with the dealer with the right truck.
When I bought my 2012 Ford F350 it was at the end of the model year (July '12) and Ford was offering large rebates. With the rebates and special pricing one dealer was $14,000 off MSRP on any Superduty on the lot on his web site. Another dealer (who was not advertising as much off) said he would meet that offer. But the first dealer had the specific truck I wanted. The other dealer (who was closer and would have been my first choice) did not.