If I understood your question correctly, the "earth" would be the chassis of the vehicle. You want the neutral of the generator firmly bonded to the metal chassis.
Its no different than "earth" (or ground or reference) point for the common car or RV (without the generator). The battery and alternators are "earthed" to the chassis.
If you mean by "earth protection" a ground fault protection then all you need is a breaker with GFP relay or trip unit on it. I would not worry about it, if GFP is not already there. Check with the mfr.