I also bought a used 2008 27FB. Mine is a Safari. I also researched everything before buying and still had problems. I'll chime in with my POV.
I did the upgrade to 3 stage converter from Best and installed myself. It is took a couple of hours to install, primarily because the instructions aren't as clear as they should be and its location on the floor under the panty which made it a bit awkward to work with. It works like a charm and I've had no battery issues and mine are about 3 years old based on the date on top of the battery.
2, 3, 4, 5 I haven't experience.
With respect to the refrigerator, we experienced the cooling issue. Here's what we've done:
1) We make sure the refrigerator is cold before loading (cool on 5 before loading). We then load the night before we leave as it will warm up again from leaving the door open while loading. It is fully cooled down to 35-38 by the time we leave. It tends to hold that temperature when parked at a campsite on setting 4. When we are on the road we put it on setting 5.
2) We freeze several small Coleman chiller packs we purchased at WalMart and put them on the top shelf of the refrigerator while we are on the road. They help keep the refrigerator cool even if the gas blows out. We do run on gas while on the road.
3) We bought one of the small battery operated cube fans at Camping World. We run it inside the refrigerator when we are on the road and stopped at a campsite.
4) I bought a cut to size synthetic furnace filter at Home Depot and cut it to the size of the refrigerator vent opening door on the side of the trailer. I put it in place while on the road. It helps prevent wind on the road from blowing out the flame. We also check to make sure the flame hasn't blown out every time we stop for fuel or a break.
5) Load the refrigerator as full as possible when you are on the road. The refrigerator will stay colder if it is filled with cold food, than if it is partially empty. If we have extra space in the freezer when we are leaving home, we pack ice from the home ice maker in ziplock freezer bags and use these ice bags to fill the space.
6) We had the street side awning added to our trailer and use it when parked. It helps keep the back of the refrigerator shaded.
7) Some people have added a small 12 volt
computer fan under the rooftop vent to help keep the air circulating behind the refrigerator on hot days. I considered doing this but so far have found the actions described above were enough to solved our cooling issue.
8) We've learned these refrigerators take much more time to cool down than a home refrigerator. Precooling to temperature is critical. Loading it just before you leave lets all of the cold air out so make sure you load the day before you leave.
9) We carry an ice chest in the back of the truck, just in case or when we need extra space for refrigerated items.
With respect to the noise in the LP tank humming, I haven't experienced it but do occasionally experience it on my home LP grill. It may be the tank wasn't filled properly and has air in it. When you empty a tank I'd suggest having it purged completely when refilling.