Update: Rolled inner right rear tire onto boards, used floor jack as safety stabilizer on axle, removed outer right rear tire to get better access to leak. By using garden hose as water source, the culprits were found to be PVC elbows on both hot and cold lines. Both had invisible cracks at the inner seam of the elbow which were spewing streams of water when under pressure. These were then replaced with stouter stuff. Testing showed only a small leak (noted before) in the shower faucet assembly which is easily accessed and will be fixed/replaced. This damage is undoubtedly due to freezing of water left in the low point of the lines.