A lot of responses while I was typing but here goes anyway. Windy, If you have a multi-meter (if you don,t you should get an inexpensive one and learn how to use it, they are priceless in these type of conditions) put test leads on the chassis battery terminal post not terminal but post. Should read approx. 12.6 volts, then have some one try to start and see if the voltage drops. If it drops significantly then your batteries are bad and should be replaced, but if it stays at above 12 volts then put the test leads on terminal on the end of the battery wire and test again by trying to start. If the the voltage then drops significantly clean them battery terminals and the inside of terminal lead of corrosion. Also then clean the ground wires the the frame. Hope this helps. Please post your results and then more trouble shooting can be given if needed.
Blessings; Keith