Many, not all 5er landing gears are by Atwood. There are several failure modes but the most common is the pin on the driven gear either backs out or shears. If it backs out, beat it back in place. You can possibly buy the pin but it is a kind of odd size and type to find. You can also use a roll or spring pin (same thing, different name) and the correct size is a hardware item in those drawers at Ace or other hardware stores.
The second mode of failure is the gears, usually both the drive and driven gears fail. Atwood sells a rebuild kit for about $30 bucks and it works well though can be a real PITA to install as you have to pull the leg to do it right including a good clean up and grease.
The third mode - the gear set bolted to the drive motor can fail. You replace that entire assembly. Motors don't fail often, luckily
The 4th mode - the long drive screw and/or the big 'nut' inside the leg will strip - with the only recourse being to replace them.
Lippert legs are similar as is every other brand - motors, gears, pins, drive screws - all can fail.
None of this is repair work is beyond the realm of a fairly competant DIYer with a decent set of common SAE (usually) tools. It is time consuming, is often in hard to get to areas and not the cleanest of projects.
Dave W along with my DW, Susan and our poodlepups, Callie & Molly,2011 Ford F250 6.7 CCLB, 5er Hitch Option w/B&W Hitch,,Ride Rite air bags, 2014 Montana High Country 343RL (38')