Not an expert, but I do have a wiring manual for Model year 2001. Depending on your model year, the circuits in question are probably identical.

The only elements I see that are common to fuel gauge and trunk release are the body electronics data bus and the rear electronics module ("REM", located behind the trim panel on the left side of the trunk). If one of those were at fault, I would expect several other failures, such as the rear headrest release, rear fog lamps, fuel filler door, and back-up lamps. Are these all working OK?

If yes, I think the two problems are a coincidence, or you did not notice one immediately. As I recall (don't have the S80 any longer), there is a security switch behind the glove box lid that disables the trunk release. This has tricked other S80 owners into thinking he trunk solenoid was defective.
There is also a fuse and a relay mounted on the REM dedicated to the trunk release, but neither have anything to do with the fuel gauge. Volvo S80 relays have been failure-prone. Fold (in and down from the top) the trunk trim panel to reveal the REM, and then listen for a relay to click while a helper operates the trunk button. You can open the trunk manually with a rear seat folded.

The fuel gauge problem could be a defective sensor or, if you are lucky, a software "glitch". Removing the main REM fuses for several seconds should force the processor in the REM to reset. Refer to your owner's manual for REM fuse ID.