It's important to have the monthly transfer appear on a Thai bank statement as International Transfer.
I transfer sufficient US Dollars but they must be verifiable as International Transfer and not Bhatnet on the Thai bank statements that Udon Thani Immigration requires as the proof source.
Udon Thani asks for source of income documentation. I also use annual Social Security letter and recent letters of Income Verification from pension funds. For me, these are easy to get from my pension providers and I usually use letters less than 2 months old