Barn is 300' from house
Electrician ran #10 wire an additional 100' from barn panel to 30A receptacle
RV is an additional 30' form receptacle
430' of wire........104V at rv.
Sorry but electrician should have known.
Remove the installed booster,,,,,,,,,,get refund. Use refund to purchase Huhges Autoformer.
Plug Huhges Autoformer into 30A receptacle...plug in your rv power cord.
Should get 114V at rv which is border line low side but OK
ON EDIT:
AFTER looking at LINK question is
WHY isn't this booster set up as buck-boost voltage controller?
Such as 115V-130V