In the past, I've "cobbled together" a 12", a 8" and a 6" quarter inch extensions, added an APEX bit holder and a 1/4" bit to make a tool. You can do it with 3/8" or even 1/2" drive extensions.
If you prefer, most any RV dealer will either have them in stock or will be able to order one or two for you. I find that for "larger slides" one is not enough.
You can also buy a replacement type on Amazon:
https://www.amazon.com/Neiko-00239A-...000RJN1GO?th=1 If you order from Amazon, you'll also need the APEX bit holder and the appropriate sized bit to fit your motor drive. You'll also need a means to attach the flex shaft to your drill driver.
Harbor Freight also has flex shaft drives:
https://www.harborfreight.com/flexib...-pc-68514.html They are about 8" long so you'd need two to reach most slide motors. At $5 each, complete with bits, using the 20% off coupon makes them a "deal"....
ADDED: Home Depot has an 11" flex drive shaft for about $12:
https://www.homedepot.com/p/eazypowe...E&gclsrc=aw.ds