Is the back end squatting because of the trailer weight or is it just low clearance overall? If it is level fore & aft but low, the best solution is larger diameter tires, if there is room in the wheel wells for them. Merely trying to raise the suspension, e.g. with air lifts, probably will not be satisfactory. And the lifts are generally only for the back end - the front is usually McPherson struts or something similar and with the front drive transaxle, the front suspension really is amenable to adjusting the height.
As Carl says, anything you do to raise the chassis will affect the center of gravity and may impair handling or stability. You won't notice it in every day driving, but if an emergency arises and you need to maneuver sharply you may well wish you had left it alone.