Paint underneath holding tank to protect the plastic?

I don't recommend painting a plastic tank to avoid the potential of solvents in the paint reacting with the plastic, either softening it or making it brittle.

The tanks under my 94 Toyota are OE and show no signs of deterioration. That is 26 years!

I do coat the metal straps with Fluid film to keep them from rusting away, any oil or grease would be better than nothing.

Paint isn't going to protect it.
FWIW, we had a TT that the tank was open to the ground and never had a problem. It was rather nice because the tank was off-white and during the day there would be enough light around it to look down the toilet and easily see how full it was!

I don't think I would worry about that. My black tank is 46yo at this point and the only repair has been from road damage.

Matt

I would be more concerned with a poorly mounted tank or road damage.

It's in perpetual shade...UV damage is all but non-existent.

Plus most paints won't stick to the type of plastic they use.

If the tank is exposed under RV than coating it with some sort of truck bed liner would be a plus..

I’d want to insulate and enclose the under belly, not paint tanks.

If they are getting rid plastic straws and bags because they don't breakdown I wouldn't be concerned about a plastic tank.

I doubt that would help. Pretty hard for sunlight to reach and deteriorate it. I've never has trouble except on one RV where the tank was poorly supported between frame rails.