Not knowing.....is the Flex (according to the owners manual)able to tow 4-down? If the book says "no"...you got a problem.
You say "sway"...do you mean the Flex doesn't track straight when being towed? Something is wrong...better find out, and fast.
If you only THINK the Flex is not tracking straight...as viewed from the motorhome mirror(s), I suggest you hook up the Flex, have hubby take the mh for a ride...and you follow in another car. SEE if the thing is really (or not) tracking straight.
Go from there...and g'luck!