while being an RD will not help you with whatever certification you choose, having the personal trainer certification in addition to the RD will make you a very valuable commodity. It is beyond the scope of practice of a personal trainer to give any SPECIFIC nutrition advice, and you will have an enormous advantage having both credentials united.
There are a lot of personal trainer certifications but I would recommend that you look at ACSM, NASM and ACE with ACSM probably my #1 for your special circumstance.
As for your CECs, I am not sure whether there are any that would apply to both credentials. I know of some CECs that apply to personal training and massage therapy. There are also nutrition specialty certifications that can be applied to a personal training certification. Sometimes it is a matter of applying to have it recognized so that you can minimize the double expense.
I wish you good luck.