I read in manual about it - not real clear on how to measure- but stopped at local cummins - big shop - and parts guys pointed me to the generator specialist out on shop floor- had questions about that and heat- and ordering the deflector to separate exhaust and intake air- he said the biggest problems are bad fuel- letting them sit and not changing oil- basically he said hes never seen heat problem in 5th wheel and if it doesn't shut of or throw errors its fine- he said the exhaust should not be a problem- he saw pics - have over 80hrs in first time out use and had zero issues - ran dual 15k a/c , res fridge , converter and tvs, sound bar, microwave etc with no issues.
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if its a concern you could run larger pipe for the added distance