I think this is a question of preference. I personally prefer native soil amended with local organic matter, but some of the 'soil' that can be purchased is perfectly fine. It may, or may not be important to you, but the traditional MiracleGro bagged raised bed medium isn't going to be organic, although I think they sell an organic option in most areas.

Keep in mind that most of the raised bed 'soils' are basically 'partially decomposed forest products' amended with compost or composted manures. They will continue to break down over time. This isn't a 'problem' per se, but unless you amend with your own compost, or other additions, you'll usually need to add a little more each year.

Some people like to treat raised beds as containers and each use different container mixes.

In short, it's going to come down to your ability, free time, how much you want to spend, and how much effort you wish to devote. Balance those issues and you'll find a workable solution.

I mix about 1/3 native, 1/3 compost and 1/3 peat and that has worked very well for me in my raised beds.

When I started my raised bed fenced-in garden (on top of bedrock) we used a local company that makes and delivers topsoil and compost. It was a sandyloamy mixture which I have been adding compost and manure to, for 25 years. I think you can start with anything and add to it each year to keep it healthy.

Be careful gardening on top of a septic field. Those things need to breathe. Usually they recommend grass over a septic field.

I second LoneJack's suggestion. Not only are these a close to perfect soil for a raised bed, purchasing in bulk (rather than bagged) is significantly less expensive. A decent bagged raised bed soil (not Miracle Gro) around here runs around $8-9 for 1.5 cubic feet, while a cubic yard of 'garden mix soil' runs about $30. You would need 18 of the bagged product for an equivalent amount, so $144 versus $30. A no-brainer in my book!!

I've always made my own using a mix of native soil and composted manure and then top-dressed with compost. I can get all those for free or cheap and by nature am frugal and would rather put in the work than the cash.

And in all honesty, I've never planted anything except grass on my septic.