In spite of your indications, you probably over discharged it a few times. 

Here is a very good manual for AGM Pb-SO4 batteries.  Other types are similar, but the voltages change a few tenths.

http://lifelinebatteries.com/wp-content/uploads/2015/12/6-0101-Rev-E-Lifeline-Technical-Manual.pdf

Pick up a decent digital volt meter.  The State of Charge and Load vs Voltage charts are near the end. 

Look for causes that might be an excess load. Check things like the pin in the safety disconnect switch. Check that there is nothing turned on, to include the refrigerator on DC power instead of propane. Check the antenna booster (although that would not explain an overnight drop that bad). Are you running an inverter for anything?

Just to note, even if a battery is not that old, it can develop a fault. I've had car batteries last only a year or two. If you can get the battery tested, do so. If that is the original battery, it is a dual-purpose battery, not a true deep cycle battery. It sounds to me like you need a setup with one 12V deep cycle battery or two 6V GC2 batteries in series,  or as in my case, a LiFePO4 battery (but that requires a lot more changes to get it going).

Get a digital voltmeter if you don't already have one. Is the battery flooded? If so, be sure that the plates are covered, using distilled water only. Leave the battery on the charger and the trailer plugged in for a few days. Disconnect the battery (-) terminal. Wait an hour, then measure the voltage. It should be around 12.6-12.8 volts. If its low , then next would be a discharge test. If its lower than that voltage then it probably has a bad cell and needs to be replaced.

For the discharge test you will need to put a load with a known current in amps on the battery for a known length of time so yo're taking out about 50 amphours (about 50% of the battery capacity rating). Let's come back to that once we get through the first step. 

The point is that 2 years is unusually short for any battery, so before replacing it you should try to understand why that happened. Did you leave it at a low state of charge for a long time? Forget to fill it with distilled water and allow the battery plates to be exposed? Let if freeze? Or just boondock a lot and go through hundreds of battery cycles? 

I really like Li batteries too, but wouldn't recommend them for the typical rPod owner who is only occasionally booddocking. Its not so much the cost of the battery itself as it is that there are just so many changes to make in the electrical system. New converter, new dc/dc charger if you want to be able to charge it from the tow vehicle alternator, relocation of the battery to the interior of the traiiler so it won't freeze, battery monitoring systems, etc. 

Did you just mention 'Capacitors'? Flux or not.. I'm a fan of. Silliness aside.. Find a slow draw capacitor and you a have product that beats all else out there... Guess we're still awaiting that 'flux' part...

(there's my dry humor again, lets see if I'm gonna get in trouble this time also)

Tony,

Why do you need a new battery for a 2-y.o. camper? 

Lead-Acid (Pb-SO4) batteries are still with us for very good reasons, one being cost.  A flooded cell Pb-SO4 battery will *generally* provide greater surge capacity, good for starting engines.  But they require water additions periodically and lose life if ever discharged below 50%.  An A_bsorbed G_lass M_at Pb-SO4 battery *generally* has a longer life, no/low maintenance and higher A-Hr capacity for a given size; they are considered spill-proof.  AGM batteries make good solar battery banks because they don't need to produce huge surge currents and are no maintenance.  AGMs cost a little more.  Gel based Pb-SO4 batteries are also spill proof, low/no maintenance and can be mounted in any orientation.  Because the gel can't circulate, they are not good candidates for starting duty and they are expensive. 

What you want is a deep cycle battery, sometimes called solar or trolling motor battery.  Their chemistry is a little different and allows discharge to 70% without undue damage.  If you buy an AGM Pb-SO4 battery, it will last longer and not need water.  I recommend the largest AGM(s) that will fit in the battery rack.  https://www.walmart.com/search/?query=deep%20cycle%20battery

My R-pod came with a 75 A-Hr Group 24 deep cycle 12V battery.  More A-Hrs won't hurt. 

ALL batteries need maintenance charging.  If left sitting a Pb-SO4 battery will self discharge at the rate of 5% to 16% a month (depending on temp).  I have a battery disconnect for my battery to keep the little bit of stuff that runs in the trailer from killing it.  Plus I added a 25 watt solar panel and a Battery Tender charge controller to keep it hot when not in use. 

A Lot to be said for Gel and AGM's, yet not experience with either (remember my friend?). Reasons for going that way (which I never did) and many have (it's new and improved). 

I left my LA's behind (goodbye old friend) and have gone Lithium Iron Phosphate as the replacement (LiFePo4). 

LA's have been the standard for years and service well if you treat them well. I'm not a fan of 12v batteries for our usage and do like 6v's paired up. Dollar for dollar, I've yet you see any other surpass the life expectancy of a good pair of LA 6v's. 

And here I am changing to Lithium... 

I don't expect this Lithiums to outlast my old La's.. I hope they can come close.. I went this way mainly for weight and having them in a better location. Well, and they go into standby so nicely, usage in the house in winter as a UPS, or reasons go on and hopefully prove out.

Your choice of battery comes from, cost (how much you wish to spend), use (what you expect to get back), what effort you wish to put into this and finally how you re-charge (amount of time required/allowed/system). 

Keep your options open. Don't be swayed by marketing. 

I love my La's (my old friend) and switching to Lithium. Take my statement as just MHO and what I'm doing. 

Like most other upgrades, how much are you willing to spend to make this upgrade happen? And why do you prefer to get away from lead acid? (other than better performance) Unless you boondock (off-grid) alot, the standard lead acid batteries are normally adequate for most campers, most of the time.

And yes, they will work on shore power converters even is there is some blurb on the label that says it will work with (or was designed for) solar power.