If bulb fits loosely in plated plastic socket, try wrapping sides of bulb with aluminum foil, gluing foil to sides of bulb, to make better contact. Adding some solder to bulb contacts on bottom and filing smooth may help contacts there.
Your plight reminded me of problems I had with an old orphan Higgins Delta Class C.I had to patch broken obsolete plastic tail light lens with a red plastic repair kit. Surprised that Winnebago used poor design/quality tail light parts and cannot supply replacements for a 2007 rig.