That's interesting because I have the exact opposite problem. Very noticeable bulge, about 18" across +/-, in the sidewall in winter but nothing in the summer. Was worried that it might be delam but it doesn't seem to get any bigger and it's not soft underneath so I don't know what the heck it is. No apparent leaks in mine either.
2013 Winnebago Aspect 27K
E450, V10, CRV, Blue Ox