How much hands on are you willing to do really? It seems you have been using a portable gen to power your 13.6 converter to charge your 6s when off grid, so you do have to get out the gen and plug the shore power into that.
(your 6s must be sulfated from never seeing any 14.x)
You are correct that you want to do 50-90s off grid on the portable gen and 13.6 on grid Float once batts are charged up (at 14.x).
To do the 50-90 you want a charger that stays at 14.x the whole time until you get to 90 and shut off the gen. At home you would first do the 14.x until the batts are to 100, then drop to 13.6 (or no power at all disconnected in storage if that is how it goes.)
So basically you want a charger that is single stage 14.x and keep the Iota now installed that does 13.6 whenever you are on shore power after the batts are full using the other charger.
So my suggestion is leave the Iota as is and get a second converter or charger that will do single stage 14.x (Trojan 6s want 14.8, eg but 14.4 would be sort of ok with most 6s)
You would get a deck mount converter and put cut off #8 jumper cable ends in its output so you have the wires and clamps same as a portable charger has. Plug that into the portable gen and clamp onto the battery bank outside and do the 50-90.
Now the problem is to tell when you are at 90 so you can stop the gen there. You can tell when you are at 50 from voltage, but not when you are still charging. I had trouble with that until I got a charger that shows amps. When the amps taper down to a certain amount (found by trial and error in backyard testing) that is about 90 SOC, then that is your marker when to stop the gen.
Another way that time2roll suggests as being close enough is to watch when the battery voltage reaches the 14.x (when amps start to taper but you cant see that) and give it another hour of gen time.
Another way is to use a battery monitor and note the amps tapering and also the AH counter for the SOC
So you do that off grid and once home do it once more only now you can use shore power to run the charger. Leave it running overnight and check the SOC in the morning (using an hydrometer is the best way to tell when they are truly full) If full, you can now unplug the portable charger-converter and just be on the Iota doing 13.6 until you go to storage.
BTW when the trailer is on shore power, the Iota will be at 13.6 and if the other charger is on the batts too at 14.x, it will be doing all the work. No problem. When the 14.x is stopped, the 13.6 is still there carrying on running the rig 12v.
So what converter choices for doing the single stage 14.x:
1. PowerMax LK model in single stage mode (least expensive)
2. PD Li model 1st generation that only does 14.x (expensive)
3. PD standard three stage with CW--does 4 hours at 14.x but you have to make it do that (you said that is too much work) (also expensive) If four hours is not long enough time to get to 100 when back home, hit the CW again for another four hour run.
On the 45 amp size, your portable gen can run that now it seems. You might need more of a gen in watts rating to run a bigger amp size charger for the 14.x job to keep gen time down. EG my 2200 (1700 really) is maxed with my 75 amp charger. You could put a 75 amp charger on a pair of 6s for minimum gen time.
You have to pick between gen time noise, and cost of a bigger gen if you want to do faster gen time for a 50-90 if you only have a smaller gen now.
