Where should I start looking for the problem on my SUV?

I was going on a camping trip yesterday and decided to take my parents small camper. I was trying to haul it with my 1992 ford explorer with a 4.0 V6. All whent well until I got on the highway and it struggled to get over 60mph. After about 2 miles on the highway I noticed my temp gauge was very hot so I got off on the next exit and pulled over to have my father come pick the camper up. Immediately after pulling over I noticed it had a rough idle almost like bad spark plugs. I let it cool for a bit yet when I started it it still ran rough in low RPMs I still went on my camping trip that was a hour away and it ran decent on the highway but when I come to a stop it sounds like a miss. When I came home this morning the temp gauge went up and stayed in one spot the whole ride home (the gauges in my suv only have a "normal" range not an exact measurement of heat). My guess is my thermostat is stuck open and I fowled a spark plug but what do you think? I checked the oil to make sure I didn't blow a head gasket and it looked normal and i don't see any leaks underneath the engine. wheres the best place to start looking?