Definitely swap those tires. They are designed for a decent ride when empty. Maybe the occasional dump run. P rated tires are not designed for real work. You might be ok with LT truck tires with a D rating. E rated tires could be overkill on a 1/2 ton. The E rated tires would be rock solid and eliminate the tires as a concern. Are you running a weight distributing setup for the trailer? If not, you need it. If so, is it adjusted correctly.
When do the Helwig helper springs engage? If they are barely engaged once it starts to porpoise the load would bounce on/off the helper springs. This would contribute to the poor ride quality.
Always drive with empty grey/black tanks.
