Not an expert on this but have a couple propane box units.
1) Get a good thermometer for both the meat and the inside of the box.
2) Yes, you do adjust the propane valve on the front of the smoker.
3) Top open and adjust side vents is normal way to regulate. But top can be adjusted also.
4) Wood chips will burn up quickly in most steel pans inside propane smokers. Switch to cast iron container. And switch to chunk wood.
5) Good chance that even on low the inside will be too hot. Several ways to fix.
a) Install a UL listed needle valve in the line
b) Can sometimes turn the valve between Off and High and get a lower adjustment just before off. I do not like doing this on a control.
But it does work in an emergency.
c) Put a wood wedge in the door to keep it open a bit and bungie it shut. Yes it wastes smoke etc. But it will save a cook!!
6) Invest in a good gasket material for the door(s) I suggest the 1/2" wide by 1/8" thick Lavalock stuff. Self adhesive and stays stuck.
7) Not all foods require water in the pan. Poultry does better with a pan full of sand as a heat sink and a slightly higher temp.
8) High temp RTV is only good for 500 degrees F. continuous and 600 degrees F. intermittent. Propane unit running on high can hit 500 easy.
Rutland furnace cement will withstand 1200 degrees at a minimum. Us it to seal corners in box etc. that might need it.
Good luck and ask questions.