New Ram Crew Cab SLT with gas engine can be had for <$45k, diesel for under <$50k. However, any Dodge/Ram 2005 or newer, deisel with under 100k miles will give you another 5+ years and can be found for under $30k. The quad cab would be tight in the back seat, but when they switched to the crew cab, it added a few more inches in the back seat making it much nicer.
Problem with the mega cab is it is only available with a short bed. This will limit your camper choices and keep their length down. Only advantage to the mega cab is the back seat reclines some.
Campers I will let others discuss because I'm only familiar with the older Lances. But if you want to keep the cost way down, look for an older Lance 1010, preferably a 2004 version as that was the best year for them. One big advantage to the 1010 which may make going old woth it is the dinette was the largest at just over 6' which makes it nice for teenagers and adults. The tanks will also let you meet your requirement. You can find nice condition 1010s for around $10k. So combined with a new truck, you would be under $60k even if you went diesel.