OK, possibly problem solved. I was crawling about under the truck today to look closer at wiring, and moved the power wires a bit and ZAP - clearly a short, started the insulation on fire even. Pretty scary. Obviously blew the fuse, so it has been doing it's job. This also probably explains the intermittent nature of the problem. So, at the very least I'll be replacing some wires, and will put them in a plastic wire loom or something like that to protect them better. And I'll probably replace the battery with more capacity (wife just started using a CPAP, so that is an additional drain on the battery overnight).
Thanks to all for thoughts and suggestions. Will make for a better electrical system overall.
Cheers,
Atlin