Depending on the size of the TV you're wanting to mount would drive my suggestions. If it's smaller (21" or so), you can get away with an vertical articulating mount similar to this one.
https://www.monoprice.com/product?p_id=7844
Note that the two mounting screws are vertical, one above the other so you basically just have to identify a vertical stud and hit it solidly.
If it's a bigger TV, something like this might be your best best, Again Vertical mounting screws.
https://www.monoprice.com/product?p_id=6198
If you go with the cabinet mount as described above, you could use toggle bolts through the cabinet and put a backer inside the cabinet to support.
If you go with the exterior wall, measure the thickness of your wall at the entry door. Then measure the thickness of the mounting plate and use screws 1/8" less than the two measurements totaled so you don't puncture the fiberglass outside.
2015 GMC 2500 Denali Crewcab 4x4
2019 Forest River Wolfpack 23pack15
Hope your travels are safe and the friendships made camping are lasting.