Based on the linked article (from May 2019), most prices seem to be 'rounded' to $1 (6.66 RBM) when paying in dollars, so by paying in RMB with smaller notes you may pay a lower price.
So small Chinese notes (1, 5, 10, and 20 RMB) would seem to be the best option.
Since the tours start from China, getting a good supply of good quality notes should pose no problem.
The $1 bill is among the most useful denominations to carry on your journey. 'It seems everything costs a dollar', is a common comment you might hear from travelers. The reason for this is largely convenience, so one need no bother with finding or making change. Items which may cost a dollar are: a soda, canned coffee, a glass of beer, or small trinkets. The $5 bill is also very convenient, although it may require the augmentation of a lone $1 to be on par with the €[$]5 note.
Also known as yuan or simply 'bi' (pronounced 'bee'), the fact that the largest denomination of 100 RMB is worth about 15 USD makes it an extremely useful currency. The 1, 5, 10, and 20 RMB notes are especially useful for making smaller purchase and notes in good condition are relatively easy to obtain.