You'd probably have best luck with a distribution block at a decent RV store. I haven't seen them with a relay built in, but I haven't looked for them either
You can find REALLY fancy - read: pricey - blocks at automotive sound dealers.
I'd probably opt for a block with individual fuses; it'll make your power distribution a little easier and a lot safer. I haven't dealt with these people but something like this would probably meet your needs:
fused distro block
You can always add an external relay to power the block.
Please keep in mind that aftermarket mobile electrics are a source of a LOT of problems - make all your connections stronger and better protected than you'd think, and it'll probably be just about right.