I have a complete Bobcat on a 2011 Tacoma, Double Cab, Short Box, V6, 4x4. I had the rear leaf spring technical service bulletin performed by the dealer. This adds a 4th leaf to the rear spring pack. Normal is 3, except in Canada they get 4. This was free, info here T-SB-0359-10 DEC 10 - Suspension - *Updated* Rear Spring Harsh Ride
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http://www.tacomaworld.com/forum/2nd-gen-tacomas/7107-tsb-list-2005-toyota-tacoma.html
I also added Firestone Ride Rite air bags. This combination worked really well for me. The truck has been down some really fun/questionable/stupid trails in Death Valley and I was pleased. I do think that the 3 leaf pack would be adequate with the Ride Rites, not sure though.
Regarding the V6/I4 question. The I4 would feel underpowered by todays standards. However, look at a Sunrader or Dolphin RVs of the past. Way heavier, less aero, dual rear wheels, and less powerful, by a lot, than the 2.7 I4. People took those all over the place, no problem.
My thought is that you would be fine if you are willing to concede going more slowly up the mountains and maybe having a hard time into a tough head wind. I think about the lower price to purchase, fuel, maintain, insure, .... a 4 cylinder, and that is a trade I would have made if I could have the 2.7 in the 4 door, 4x4. They don't make it.
One important detail, which I have yet to address, is to make some kind of fairing so that the camper is just not scooping air at highway speed. There is a lot a space between the roof of the 05-12 Tacoma and a Bobcat. It is very turbulent and I honestly feel and 60mph+ an I4 with a fairing would give the same response as a V6 without one.
Ok, one last thing, then I will be quiet. The turnbuckels that hold the camper to the truck, they came lose. They are totally sufficient for 99% of driving. However, on tough trails like the Lippencott in Death Valley, I think some type of spring loaded or "elastic" turnbuckle would be helpful. I am not sure if this is a Tacoma thing, a Bobcat thing, or a physcics thing, but something(s) was flexing and stressing the buckels a lot.