Help Me
You're quite correct. It is a complicated matter.
Look at the manufacturer's ID plate on the Isuzu's driver's door jamb. (Maybe it's somewhere else close by.) You should see something listed as a GCWR (Gross Combined Weight Rating). This is the maximum that your entire rig can weigh in order for you to tow the trailer under normal driving conditions. It includes literally everything that you carry in the Isuzu and the trailer: you, the spouse, the kids, the dogs, gas, propane, water, sewage, food, pots & pans, clothes, rock collection, fishing tackle, kid's bikes, school books (the kids ARE going to do homework on their vacation, aren't they?), other play-toys, etc., etc., etc. EVERYTHING!
Your safest bet is to scout out a public scale long in advance. These are run by your highway department, major trunk stops, cement and gravel companies, major meat packing plants and several less well known outfits. Watch for signs along the highway. Look in the Yellow Pages under "SCALES." Stop in and ask about how to use them a week or so before you plan to leave.
As you leave on your trip, after everything is packed including you and the spouse and as the very last item on your list, stop at the scales and have your entire rig weighted. It may cost $10 or so, but trust me, it's worth every penny. You will then have an authoritative number for the weight of your entire rig, loaded. At that point, if you weigh more than the GCWR you need to return home and start unpacking some of the less important stuff that you thought you couldn't live without. Then go back and re-weigh.
Otherwise you head out of town with a smug grin on your face!
There are lots of other things you also have to think about, like tire pressure and the carrying capacity of the trailer, but those are different issues.
