The valve action of shocks is a little different with towing . Not necessary firmer . Just tighter on rebound to prevent this action . My old work Rideo gave me car sickness when I was driving ! Not towing anything . My 8m motorhome did the same on highway in WA . I check std front assist air airbags ? Pumped from 10lb to 50lb . In this case problem solved . I can see where load levelling device can be too tight creating a whiplash effect as with single - double non sharing van springs . Some times it's the action of variable Spring rates on coils springs too .