tomato plants failing

What size pot? Your plants may be completely root-bound.

In my experience, the most fertilizer for potted plants is Osmocote (or a similar 'off-brand' product). It's an encapsulated, slow-release fertilizer that can't be easily leached out of the pot.

Drill some holes in the base of the pot (if there aren't any there right now). I've never had to worry about potted plants get overwatered before -- usually, the opposite is the problem. (Last month's record rainfall only meant that I didn't water the couple of potted plants that were out there at all.)

Mulch works with potted plants as well, Pat. I give them as many wood chips as can fit on the pot (or compost, or manure).

Mulch isn't enough for a large, heavily planted pot in August. It just isn't. I end up watering the pots nearly every day. And these are pots that are made of that foam material so they insulate the roots inside. Of course, these are strictly ornamental pots. I mulch mine with cocoa shells and shredded leaves.

And flats I may have started (waiting for space to open up in the veggie garden) can need watering every day at a minimum, once the temperatures creep up to 90 (like yesterday).

Let's say it cuts down watering by 33% depending on the plant. Shiny leaved plants fare better than, say, peace lily. It is not as good as mulch on soil, yes, but I leave my houseplants outside while on the road for a week at a time. Necessity is the mother of experimentation.