Corcomi, fwiw, I've never had a manufacturer give me a hard time about parts and warranty labor, and I have never filled in a warranty card for any if the equipment. Norcold, Dometic, Alterra, Fantastic, among others, simply asked for proof of the date the rv went into service. Including the second year from Norcold, Alterra, and Dometic. Some were parts only, but went beyond the one year that Oliver wraps. Truma could be different, but I would doubt it.
I think the cards are more for people who buy assemblies as replacements. Even then, a receipt has been enough...in my experience.
Hang in there. Most of these manufacturers really do care about their reputation and goodwill in the marketplace.