I go along with the weak battery theory. Is the rv plugged in to shore power. Try plugging in and see if it makes a difference. Shore power should also charge up your battery.
To properly test an RV "house" battery (the one that powers stuff like inside lights, water pump, furnace motor, etcetera), the battery needs to be at rest. This means not charged within the last hour or so, and with nothing drawing power from it. Since you’ve now read your multimeter instructions, hook the probes up to the battery (best to observe polarity), set the meter from the appropriate voltage range, and check the reading. Now compare to this information:
Volts --- Remaining Capacity
12.63 --- 100%
12.54 --- 90%
12.45 --- 80%
12.36 --- 70%
12.27 --- 60%
12.18 --- 50%
12.09 --- 40%
12.00 --- 30%
11.91 --- 20%
11.82 --- 10%