The transmissions in the E450 got better in 04 or 05, and there are some vedors that offer programmers that improve shift patterns and lock up logic, such as 5 Star.

The tow rating from the RV mfr will be low, based on the receiver being attached to the lightweight extended frame that supports the house behind the Ford chassis. This is one of the reasons that I specifically looked for a Class C that had a smaller frame extension, and a longer wheel base - so there was less light duty frame to reinforce, and less lever arm for the trailer to push on the rear axle (vertically or side to side)

Dig up the GCVW data for that E450 cab and chassis to get an idea of what you could tow IF you set up a properly reinforced hitch. My 05 shows a 21,450 or 22,450 GCVW, while the RV company equipped it with a 3500 hitch. We reinforced the rear and towed a 26' enclosed over the rockies and back last summer no problem. Probably was about a 6000-7000 load on the return trip.

The plug issue in the V10 was corrected earlier than 03 - I think. But double check that to be sure.