Unsigned bills should be acceptable as that is the normal practice followed in petrol pumps. Even if they aren't (worst case scenario) the one signed bill is sufficient to establish that you are a consumer u/s 2(d) of CPA.
Regarding the avenues, both
RTI and CPA are independent laws not conflicting with each other so you can even continue with all 3 avenues together. However looking at the facts of this case I personally feel that
RTI would be a long battle and even after getting all the information to prove the guilt of oil companies you will have little powers to get anything enforced on them. This is why I suggested the CPA route. As repeated earlier, you need not stop the two
RTI applications for filing case under consumer court.
True that documents obtained under
RTI Act will help you in consumer court but getting the documents before approaching consumer court is not compulsory. All consumer courts have powers of civil courts and they can order the company to produce all relevant documents.
To proceed with CPA you will first need to send a written complaint to the company and if you are not satisfied with their response then you can proceed to consumer forum. Let me know whenever you want to proceed with this and I will send you the details of the format and procedures etc. However do bear in mind that for filing a case in consumer forum their is a limitation period of 2 years from the date of arising of the cause of action after which complaints will not be entertained.