The distance between remote and door opener has progressively gotten shorter until now it is only intermittently working even standing under the door opener. I've replaced the battery twice now; the first time made little or no change but it was still working if you were wishing about 25 ft. I suspect that the battery is not the issue but rather the receiver on the door opener. Is there any adjustment, cleaning, contacts, etc. that can correct the problem? There is an external power supply transformer and an antenna as well that might be the issue? Any suggestions appreciated.

Are you capable of simple troubleshooting?
1. Can you check the voltages of the transformer, both input and output?
2. If those are in order then check antenna connections.
3. Reprogram your remote.
4. Replace the remote.
5. Replace the receiver.
Troubleshooting is simple... what you can't fix you need to replace.