A 2003 of that size will be a W22 chassis. That means 22,000# for the MH at max load. The total weight capacity is 26,000. That leaves 4000# if you are at max load in the MH. The 5K hitch may be just the hitch and not the total allowable towing. The W24 didn't come out until a couple years later. There are different brakes on the heavier chassis also.
What kind of brakes do you have on the boat trailer? I had surge brakes on a boat trailer that I towed a couple of times with a MH. That worked OK but launching with the rear overhang on a steep ramp was something else.