If you really want a power-dense engine, teh two stroke opposed piston opposed cylinder is an interesting option.[0]
As for range-extending trailers, Tesla cofounder J.B. Straubel had it right. The last time I linked to his "pusher trailer" the response was surprisingly close-minded for HN. The reasoning was (paraphrased) "I don't believe it because I don't trust the builder that it works because I don't believe it."[1]