Mitsubishi Challenger 2015: Is it a lemon?

We had to abort our caravanning holiday to the West Coast of Tassie when our 2015 Challenger could not tow our 1.49-tonne (tare) 6-metre pop top van in hilly country without overheating the automatic transmission, or the engine or both, causing frequent stops. The engine was also prone to pinging under load. After the engine coolant finally boiled we gave up and returned home, slowly. The caravan carried 130 litres of water, basic camping equipment and a week's rations. Its gross mass could not have exceeded 2 tonnes. The wagon carried two adults and a dog, 120L of fuel, 20Lof water, basic recovery gear, and a small toolbox (say 400kg max) giving it an estimated GVM of no more than 2.5-tonne. The estimated combined mass was therefore not more than 4.5 tonnes, well below the 5.65 tonnes specified maximum. All fluid levels were checked before departure, the tyres inflated to 36 psi on the wagon and the rear air bags to 20 psi. The caravan has tandem wheels with tyres inflated to 35 psi. It is well balanced, level and has always towed nicely. At no time did any warning light display. The transmission would simply go into "limp mode", dropping into second gear and requiring an engine restart. The engine temperature gauge needed to be watched as it often rose above the mid line. After the engine coolant boiled its level was checked again with no apparent leakage. I do not consider the Smart Bar, low profile driving lights or bash plates create air flow problems as our 2012 PB Challenger was identically equipped, plus a snorkel, and never had a problem with overheating. Does such a problem rate as a major failure? Is it sufficient to claim lemon status?