Appreciate advise.
I have to spent some time in marina so I will use tap water.
I know that tap water is not good for RO flashing. I have water from water maker in Jerry can.
Can I flash this water in closed circuit?
Jerry can to water maker, water maker back to Jerry can?
This way I won't use tap water for flushing the system.

I use to have an echo tec water maker. I have copied a link for the manual. It explains cleaning the membrane and flushing. Maybe it will help with yours.

www.echotecwatermakers.com/pdf/pamphlet/DML-200-260-Manual.pdf#zoom=100

Biggest issue with using a freshwater flush is not using chlorinated water, otherwise you can ruin the membrane. What you are suggesting sounds fine, so long as you are ok that the water has not got contaminated can you run the water through some filters first? Most flush systems use a charcol filter, then you would be fine using tap water.