Lead-lined could mean anything. Might be sheets of lead foil hardly thicker than Reynolds Wrap. Doesn't take much thickness; a mere 1mm thickness will block 99% of x-rays.
The problem is, lead is a carcinogen. If the owners kept the RV, it would cost them a lot more than the vehicle is worth to have the lead removed, so they're trying to sell the problem to someone who doesn't know what he's getting into.
Engineering: Finding complex solutions to simple problems you didn't even know you had.