In terms of clamp meters, Home Depot has a good selection. I bought an A/C meter but opted for an A/C-D/C meter later, it allowed me to check draws from the battery and from individual wires in the RV.
Here is a web link: Search Results forĀ*clamp meterĀ*at The Home Depot
To check the amperage draw for the entire RV I put together a short extension cord and I opened the outer sheath on a small portion to expose the insulated black and white wires (but didn't take the insulation off!), you need to clamp onto either the black or white wire to read the amp draw. I used a 50 to 30 amp adapter then a 30 to 15 amp adapter to plug into the extension cord so I could read the draw. You could do something similar with a 30 amp extension cord, but you would need to check both legs if you you used the 50 amp cord.
Mexico was great until the violence level went up in 2010 due to the druggies. We stayed on the Pacific coast in campgrounds around Teacapan (2 hours south of Mazatlan) and south of San Blas which is north of Puerto Vallarta. There are a number of campsites all along the coast.