If you are measuring at the battery posts with the DVM and the battery *is* going down to 5V .... the battery (or batteries) have a problem. That seems unlikely with a new battery, but stuff happens.
More likely, you are measuring a bit away from the battery and have a wiring issue. An open / poorly connected ground is one very likely issue. More or less:
14.48V on the converter = converter is up and running.
14.48V on the battery posts = battery is connected to converter
= battery is fully charged
5V under a tiny load at battery posts = battery is dead
Since the battery should be up in the 100AH range and your load is a couple amps, a fully charged battery aught to keep going for 10 or 20 hours. At that point the DVM should read around 12.1V or so.
So what to do:
1) Check on the battery posts with no load / no charging, you should get 12.6 to 13.2V. If it's below that, charge the battery.
2) Check on the posts with the fan running (about a 2A load) and see if the battery drops to 5V. If that happens, you have a defective battery. On the posts = directly on the lugs on the battery, not on a bolt or a cable attached to them.
Assuming the battery actually is ok:
3) Start from either battery post and work away from there. (with the fan running). Keep the DVM lead on one post and move the other lead along the connection chain away from the battery.
4) Repeat with the other post.
At some point along the wiring chain, you will go from 12V
to 5V in your reading series. It's a good bet that whatever that point is ... it's a bit hot.
Since this *could* mean using 50' long DVM cables (and arms to match) there is an alternative as you get away from the battery:
Find a clean spot on the shell / frame and measure both posts to that point. Hopefully the negative will read near zero and the positive around 12V
. If the negative reads 5 or 6V then that is a *very* good indication that you have an issue in the negative side. Then as you move away from the battery, take your readings to shell / frame rather than to the battery post.
Be sure to include the DC bus arrangement (however they did it in "your era") as well as the DC fuse (probably not breaker ) panel as you do the measuring.