As others have said, the proof of income won’t matter. What is more beneficial is using FMP (foreign medical program) for direct billing to the VA. In this case, you’ll have no out of pocket expenses if you’re testing a service connected disability. This is normally only done at a few of the large private hospitals. There are 5-6 VFWs in Thailand. You should get in touch with the local one and they’ll help advise you on that.