If you go with 2 12 volt batteries, you will double the amps available. Connect them in parallel.
You have approximately 48 amps with your 1 battery at the 20 amp hour rate. 2 Batts will give you 96 amps. However, remember, you should only draw them down to 50% or less to avoid damage. So 2 batteries will give you 48 amps drawn down to 50%. The reason you don't have many amps in that battery is it's not a true deep cycle battery. Marine and RV "deep cycle" batteries are rarely rated using the industry standard 20 amp hour rate. They are either estimated or not mentioned at all. Go look at a GC 2, 6 volt battery. Most are rated for over 200 amps at the 20 hour rate. 2 6 volts in series gives 200 amps at 12volts. GC stands for golf cart. They are true deep cycle batteries.
What does this mean to you? Well... Each 12 volt light bulb you run uses about 1amp per hour of use. Your fridge electronics, built in C02 detector and the like each draw power all the time. If you add up the power used in your rv, you will find why so many rvrs go with multiple 12 volt battery banks or multiple 6 volt batteries....To give them enough power available without damaging their batteries by drawing then down to far.
Apologies for long post. Search the archives, the internet for further explanation. Short answer? Get 2 batteries. Get the same battery for#2 as you already have. It's even better to buy both batteries at the same time. Same age, same capacity.
2015 Sierra 1500 Denali
2014 Coleman 249RB
David Jon Devoucoux
People, Places & Things...for over 40 Years!