Firstly: Is it appropriate to use multiple adverbs in a row? Yes, it is. Normally I would expect them to be separated by a comma or "and". For example:
I slowly, stealthily, went downstairs
I went downstairs slowly and stealthily
Notice that these adverbs both behave prototypically: they (separately) modify the verb.
Secondly, Are "fully" and "electrically" adverbs, despite "powered" being an adjective? Yes, I agree with your understanding, that adjectives, including participial adjectives, can be modified by suitable adverbs.
Thirdly, the actual example given: "This car is powered fully electrically". Well, it does sound rather odd worded like that. I think it is because "fully" refers, in this case, to "electrically powered". (It would mean something different to say that a car is both "fully powered" and "electrically powered".) Normally you could have the adverb before or after the adjective. So it sounds right to my ear to have "This car is fully electrically powered" or "This car is fully powered electrically", but not the phrase that you came up with.