Ignore everything else, and just charge at 14.4v until the amps to the Lifeline AGM get down to 0.5a/100AH of rated capacity.
That is the ONLY indicator of "Full" that counts. Once there, you can "zero" or "synch" your monitor(s) to zero amp hours, and let them run from there.
BTW Lifeline also wants you to charge at a Minimum 20% in Bulk. 255AH means it wants a charge of 51 amps until Bulk ends and amps taper. That will be at about 75% SOC, so if you don't draw down to below 75% you can't ever meet that20% spec. Mex says that doesn't matter if you start charging at above 75% as long as you get the amps down to 0.5/100 (so with 255 that would be 1.3 amps)
So you need a charger that will stay at 14.4v as long as needed until amps get down to 1.3 amps. Many converters and chargers won't do that. (A PowerMax LK model will though EG)
Your solar 300w will do maybe 15 amps flat on the roof at noon for a little while so no way to meet that 51 amps spec. If all you have is solar you need to stay well above that 75% SOC by morning and have a long sunny day to even hope to get down to 1.3amps by dark and before you use anything 12v. Not likely!
So you need a gen and a charger that can do at least 51 amps and get to past 75% before turning the job over to solar if there is any to be had that day for long enough.
BTDT, just how it is.
1. 1991 Oakland 28DB Class C
on Ford E350-460-7.5 Gas EFI
Photo in Profile
2. 1991 Bighorn 9.5ft Truck Camper on 2003 Chev 2500HD 6.0 Gas
See Profile for Electronic set-ups for 1. and 2.