the cheapest way to add power is :
1. jumper cables
2. an extra RV battery
3. an extra tow vehicle battery ( if you only have one )
gm diesels have 2 batteries .
2 batteries are optional on gassers .
you can add the additional tray and a battery to a single battery gm.
4. when replacing your tow vehicle battery go to the AGM battery such as the exide edge or johnson controls optima .
5. Although 2 identical batteries are BEST for your RV , if you only have a small deep cycle , you can still add a group 27 or group 31 deep cycle and gain a little extra over an additional group 24 .
6. the cheapest time to add amp hours is at replacement time . Get the biggest batteries that fit .
7. even if you decide to add only a group 24 , go ahead and get a battery box for a group 27 . That way in 4 years you only have to buy an additional group 27 box.
8. Charge your batteries to 100 % on 120V before you leave home .