Float charging AGMs while under load
This is a valid and difficult question.

My suggestion would be


A DC-DC boost charger to charge the batteries from any DC source. If you're happy to just float them, try eBay for a switching PSU board.
Schottkey diodes from both Bat+ to Radio+ and PSU+ to Radio+.
A relay to prevent the battery from being used, at all, when the external supply is connected.
A "direct battery" connection point for your external charger.


Two diodes are needed to ensure that you don't back-power the supply, and finally, to make it possible to detect the external power failure, which you can't do if there's only one 13v bus.

The relay is needed because under full load the external supply will certainly droop below the voltage of a lead-acid on charge. A simple OR gate might not be what you want then.

The switch-mode charger might create a lot of RF noise... You can try to filter it...

Consider using two 6V batteries, and switching them with a relay: parallel for charging from an unknown source, series for supplying the load. Then you can use a linear charging circuit, current limited constant voltage.

Last thought - don't skimp on the cables and connectors. With 20A, every fraction of an Ohm counts. On my HF radio, the wire supplied with the radio suffered a 2 or 3 V drop when transmitting full power. Do the sums, go one gauge heavier than you think you need, and use decent connectors. This is where you'll lose the volts, not in the schottkey diodes.

Edit: seems I've designed the SuperPowerGate, and maybe improved it...