Caution about the Sprinter.

Hello,
I'm new to this forum, so hi everybody!
I want to share a word of caution.
We just bought a TM 2720sl. With air conditioning, it weighs at 3198 lb.
Our tow vehicle is a 2004 Sprinter van. It has a tow rating of 5000lb, so we figured we would be OK.
We loaded the TM with the usual gear: clothes, pots and pans, towels, a microwave and a coffee maker, not much else.
We loaded the Sprinter with the rest of our gear: most notably, weight wise, an inflatable kayak, a bicycle and a tricycle (adult size).
Our Sprinter is a cargo van, with only two seats. A year ago, we soundproofed it. The soundproofing is fairly heavy.
To make a long story short, we left North Georgia, headed to the hills of North Carolina. The plan was a two month trip to Quebec.
That was yesterday. Today we are back home. The Sprinter literally could not make it up the first extended hill we faced (North of Franklin, toward Asheville). It started slowing down severely, and it was clear that it may not make it at all to the top. Scarier yet, we were wondering how it would have fared on the other side of the hill. Luckily, there was a rest area where we could stop, make a u-turn and head home.
Now we are weighing everything (should have done that first!). It turns out we are at about 4800 lb between TM and cargo. This is below the 5000 lb rating. However, it is clear to us that the rating is only valid on flat terrain. I do not know what the multiplier should be for mountain driving.
So, lesson learned: the tow rating and GCWR are not enough to judge if a tow vehicle will work.
I would love to hear suggestion as to what vehicle would be appropriate for mountain driving towing the TM. For now, we will only drive on flat terrain. But next year we might be able to afford a new tow vehicle (out of the question this year, unfortunately).
Hope this helps anybody who is considering the Sprinter!