When you find the pump, clean out filter, turn on a faucet and see if pump runs. If it's the original pump, it may need replacement. If you want to upgrade to a higher flow rate pump, your wiring might not be heavy enough. If your rig has a little Intellitec pump controller device (probably located near the pump), you should be OK installing a bigger, higher amp draw pump. I recently installed a 5.3 gpm rated Remco 55 ARV model pump. Cost around $170,great water pressure for shower, etc., but noisy.
http://www.ebay.com/itm/Intellitec-RV-Water-Pump-Contol-Unit-00-00145-100-/321238744329?_trksid=p2054897.l4276#ht_717wt_984
http://www.amazon.com/Remco-Aquajet-5503-AV15-B636-Pump-12VDC/dp/B00571JK6W