My vague understanding of the Thai ambulance system is they use the pickup trucks in rural areas or 'first response' units. The rescue teams are all quasi-volunteer organizations that manage to make a profit and have full time employees. The hospitals 'donate' to these rescue companies and it's probable you'll pass a hospital that isn't so generous to get to one that donates more.
I've been passed by these pickup units in rural areas, only to see them in the next village on the side of the road transferring the stretcher to an actual ambulance.
I grew up in an area of the US the nearest 24 hr emergency room was 30-45 minutes away by car. The County PD had a few 'crash wagons' positioned around the county at any given time. (Police cars that were large Chevy or Ford station wagons, with a stretcher in the back.) Actual ambulances were spread out much thinner, and no helicopters back then. It was pretty common to die in the back of one of those on the way to the hospital, or get into a second wreck.