For DIY: Try searching for cabinet drawer slides. You will need to narrow it down to heavy duty, of course.
Here are a few pointers. These are to some manufacturer's websites. You will probably need to find a retail dealer for these. One type of place that *may* sell to individuals are cabinet supply houses (distributors). If they do, most of them are very knowledgeable. Just remember, they are in business to serve their commercial customers. So, if they are really busy at the counter, you might try returning when they have some more time to talk to you. It's nothing personal. It's just good business sense.
Also, the local distributors may NOT have these extra heavy duty and long sizes in stock. They may need to be ordered.
You many want to look around some of these websites as they offer a variety of different slides for different purposes.
Here is a heavy-duty locking slide from Knape & Vogt that will handle a drawer up to 48" wide and 60" long. This one mentions RV Vehicles as a possible application.
Another major player in the slide business is: Accuride. Here is one type.
There are probably others, but these are the two big companies that come to mind for this type of heavy duty application. I am pretty sure that you are NOT going to find what you need at a big box store.
Normally drawer slides mount to the left and right sides of a cabinet box. However, some companies offer accessories to adapt their slides to act as a "rollout" and mount to the floor of a cabinet box.
Whatever you do, be very sure that the slides are very securely fastened to the basement. Otherwise, when fully extended and with a heavy load, the entire assembly could rip itself right out. It sounds scary, and it should, but by simply securing it properly this can be easily avoided. In other words, don't expect a couple of 3/4" wood screws into the basement wood floor to hold 500 pounds of your stuff.