Hi Bob, my 25 cents worth (might be worth less though).......
1. A 3/4 Amp trickle charger should not matter if you just connect to one battery that is paralleled to another battery which is co located (short cables). At that current there is barely any voltage drop across cables to be a concern.
2. Tenders are single stage float voltage trickle chargers and should be enough to keep charged batteries topped up for short term storage. For long term storage using your coach multi-stage converter and shore power to keep all batteries charged would be a better option IMO. Modern multistage converters cycle wet cell batteries to help prevent plate sulphation.
3. If you are in storage mode I assume you turn off your coach and chassis battery latching disconnects. This would mean they are not connected to each other (other than common chassis ground). So likely a second tender would be best assuming you don’t do the coach converter and shore power option. There are optional devices for charging both sets of batteries........but I am not familiar with them. Look down the attached link and there is one described there as well as trickle chargers.
https://rvshare.com/blog/basic-rv-ba...arger-options/
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