I am sure you are aware of the Qualified Retired Persons Program (QRP), which provides many benefits if you are going to make Belize your home. We banked with Atlantic Bank in Belize and also retained a US account. You can check on the requirements for the QRP program (if you have not already). If your retirement funds were going to a US bank, you would only have to set up a monthly automatic transfer to a Belize bank.  US credit cards are accepted just about everywhere. US checks are very difficult to use in Belize as the banks put a 10 day hold on them. We loved Belize. Be sure and rent for six months or so before you make a commitment just to "make sure". It is very different from the US. As much as we decry fast food in the US,it does not exist in Belize.When we crossed the border into Chetumal, we made a bee line to McD's! Corozol is close to the Mexican border and offers many amenities in Chetumal that we did not have on the Peninsula, while still retaining the charm of Belize. But, if I had it to do over, I would still live in the Maya Beach/Placencia area. I agree with CaribeGal about the smaller places. They are cash only. But you won't find that to be a problem. And with the 2 to 1 exchange rate, you will feel like a King.
One thing to be aware of. If you put your US dollars into a Belize account, there can be a penalty when you withdraw. We kept most of our money in a US account and just transferred to a Belize bank our living expenses. We also kept an emergency savings account.Â
Should you require an emergency medical air lift back to the US, it is usually a cash deal.(If you think that might be a problem, look into the insurance). However, we were well pleased with the medical care and would recommend Belize Medical Associates. There are excellent hospitals for more complicated medical procedures in Merida, Mexico, but be aware that US insurance does not cover you there.
We kept our US health care insurance "just in case", but never used it. A visit to a qualified specialist, for example an Orthopedist, is $60 Belize or $30 US. It is not unusual to call up and be able to schedule an appointment for the next day; often you can get in the same day. It is a whole different way of living.Â
Loved it there. Go, travel around, enjoy.