Second leisure battery

The link leads between the batteries need to be the same length and gauge. The main neg. and pos. taken from diagonal opposite terminals of opposite batteries to the charger/load don't have to be the same length. The resistance will balance as it becomes just one battery with a series internal resistance for intents and purposes.

If you have a fully functional starter battery for a house battery after 5 years I'd say work away with the tractor battery you won't see the use to have huge implications, that or your meter is seriously out of calibration.
I'd discharge test the old one to learn it's true capacity and then get one to match. (aim for 50% discharge in 10 hours with a fixed load like a bulb...let the battery rest 24hours if it's sitting about 12.2v or 12.1v note your capacity (discharged amps) double it and buy a second one with this many Ah). If it sits higher discharge for another hour and then wait another 24.

There's no danger of the world ending, starters just don't like deep discharging and therefore won't last as long as a semi-traction...that is all. They've thinner plates that get corroded faster. Tractor batteries are fine for light occasional use. Plus they're so cheap you could just keep replacing them and still come close to a comparative price per watt.
The reason batteries are best kept same age, make and capacity is to prevent one discharging into it's partner. 1 weak battery in a bank of two = two x weakest battery.

Battery bank balancing.

110ah / C20 rate = 5.5Ah discharge for the rated curve.
12.4V x 5.5A = 68.5W

So hot-wire the high beam of a headlamp to the battery and let it run 10 hours (60W / 55W...H4 type, 70W would be better because they're not efficient...you're only wiring the larger filament...you'll have two pos. and one neg. just use the blinding pair).

Use some decent cable too, min. diameter 1.5mm or ya might melt it.

Best check it periodically. I'd stop the discharge if you see 11.8v and let it recuperate a few hours. Chances are it's not a 110Ah battery anymore.

This isn't a perfect solution by the way. The old battery will have a higher internal resistance due to residual hard sulphates but it'll get you pretty close to where you want to be I reckon.
I'd look at solar after 4 days run-time.
Two days sounds a little low for 110Ah, you could possibly double your run-time without the inverter and a 12v tv. But then again another battery is probably cheaper.

It's like them old tungstens inefficient in terms of light per watt but much more respectable if you consider them a light and heat source. :pac:
I bet a CRT would keep a camper right toasty.

You shouldn't water a battery before you charge it, the electrolyte expands when charged and you can get a spillage. In your case you did the right thing and got the level above the plates but that is as far as you ought to go until it's charged then top it off.
Damage was done before you watered it. The plates are oxidised. It's irreparable you could try filling it with caustics but I wouldn't bother. Scrap lead, 50c per kilo.
There's a possibility the lower electrolyte level was more concentrated so worked better for the reduced capacity you had.
[Edit]: Stratification is another possibility, it's an unequal distribution of acid concentration, this raises the open circuit voltage but causes increased corrosion and sulphation in the lower end of the battery and overall reduced capacity. This happens when the electrolyte doesn't get mixed often by driving, charge cycling, equalising etc.

You can try equalising it and see if it recovers but I'd say it's a ~60Ah battery now if you get all 6 cells working.
Bubbling below 14.2v means dead cells or soon to be.

You can check the cell voltage with long nails between cells, just stick them on top of the plates and meter them, use the neg. post with the first cell to get that one if you want to compare the voltages (be aware this will contaminate your electrolyte).
Or you could use a hydrometer to check the electrolyte density and work from there.
Either way it's just going to tell you what you already know I'm afraid...new battery time.
5 years is impressive for a starter come leisure battery.

On the bright side if you follow the original plan to get a new battery you might find that a genuine 110Ah meets you requirements for the duration and remember to check the water levels more often.