As an example, this site shows the battery dimensions as well as amps.
When you measure the battery box on the camper, measure the opening, not the box, it might have a smaller door .
If you have to, you could always mount a battery in the bed of the truck,in front of the wheel well(s) .
This link shows AGM batts but a deep cycle wet cell can be used with VG results for less money.
http://www.dekabatteries.com/assets/base/1740Intimidatordeepcycle.pdfI have info, regarding the amount of power that is used by the different appliances in an rv . If you want it, i'll look for it for you.
Depending on your use and needs one battery might work , might not .
I do not let my deep cycle batts fall below 12 volts or approx. 50% of capacity, it can damage or shorten the life of a battery over time . Use a volt meter to monitor the battery as not to deplete it.