Personally, I suspect that Billy hopes and prays he doesn't end up in the White House with President Hillary. I think he'd be much happier avoiding the hassle and public scrutiny that would go with being the First Dude.
I wouldn't be surprised if in the privacy of the voting booth he actually votes for Trump.
I think the only reason he campaigns for her now is that he's too old and feeble to risk any more whacks in the head from her thrown ash trays and lamps if he doesn't publicly support her.
Lol.