Hi PapaRoe,
Your 0.56 and 0.26 VDC readings makes me question your readings. Although a direct short could completely drain a battery, there probably would be fuse protection of any high current drain.
Here's what I would do. First, using your same meter in the DC position, check a known good battery such as the one in your truck and see if it reads around 12.5 V. Then retest your RV battery. If you find those same RV battery results, under 1 VDC, disconnect the battery from the cables that go to everything in the RV and check the resistance on those cables (black for ground and res for positive) with the master switch (next to the raise/lower switch probably) in the off position. I think you should measure an open circuit or very nearly- several thousand ohms or no reading. Can someone confirm this? Next I would turn on the master switch, but without any lights or appliances turned on, and see if the resistance changes. Each light or 12 V appliance you turn on should reduce the resistance reading. If, with the master switch in off position or in the on position and no lights or 12V appliances turned on, and you read a low resistance like 5, 10, or 20 ohms or lower, then something is drawing current, and I'd have to study your electric circuit to see what it could be, possibly the hydraulic lift motor or the converter circuit.
The suggestion to remove all the fuses and then replace them one by one to isolate the circuit with a short is a good one. Again, when you read low resistance it indicates a "load" which could be a shorted circuit. When your meter shows no indication of resistance or thousands of ohms, it indicates a open circuit (or nearly so).