As someone else said - the floorplan is probably the single biggest factor in knowing what you want. I love ours
except I can't easily get to the fridge with the slide in. It's more and more of a nuisance as time goes on (and we've only owned it a year now).
Otherwise, for the truck - there are 2 schools of thoughts on this and each party vehemently demands that they're right and the other is stupid:
1) You must stay within all of the ratings for the truck - this includes the maximum the truck can carry (aka gross vehicle weight rating or GVWR), the axle weight rating (GAWR) and the tire load capacity and finally the gross combined vehicle weight rating (GCWR) (i.e. the weight of the loaded truck and loaded camper).
2) It's acceptable to exceed some or all of your weight ratings - for instance, as long as you're within axle ratings and tire load capacities, you're safe.
3) Ok, I said 2 groups, but the 3rd is those that say, "it tows and feels fine, therefore I'm OK".
Personally, I fall into the first group and feel you should stick within your ratings. I understand the reasoning and logic behind the second group. The 3rd group, well... to each his own, I guess.
NOW- how do you know if you are or are going to be within your ratings? A little bit of art, a little bit of science.
Since you don't have a truck, the easiest thing is to pick the trailer and then work backwards from there. I'm a fan of doing my calculations based on the maximum weight the trailer is designed to carry (it's gross vehicle weight rating). More often than not, you wan't get anywhere near this number - but it's good to have in your back pocket. Most of the time, depending on the size of the fifth wheel, you'll be anywhere from 1,000 to 2,500 pounds over the completely fictional advertised "dry weight".
Once you have a ballpark estimate (or go fully risk adverse and use the maximum weight), you need a truck that can carry 20%-25% of that weight as part of it's payload
in addition to you as the drive (whatever your weight is minus 150 pounds), your passenger(s), your pet(s), your stuff in the truck, your add-ons to the truck, your hitch and anything you might store in the truck bed. As you can imagine, that payload number gets eaten up pretty quick. For me, I found that myself, my family, my truck add-ons and our stuff weigh about 1,200 pounds. I then add my fifth wheel's pin weight (the 20%-25%) is about 2,300 pounds (mine is actually 16% of my loaded camper's weight of just under 14,000 pounds).
It's all a bit confusing. See if these two write-ups will help you out:
What Can I Tow?What Can I Tow? A real world example