You want to be sure tires, wheel
bearings and brakes are all in tip-top condition. A failure on the road is no fun. I do all this every year, after a 4,000 mile summer trip, towing 12,000 lb.
Look over the entire trailer carefully. Anything that looks worn or damaged may need to be addressed. Check the tires all over, including the spare. Worn or uneven tread, bulges, or cracks in the sidewalls are bad signs. Make sure they're not too old (> 4-5 years) even if they look good.
Wheels should be balanced, and tires inflated to max pressure listed on sidewall, unless their weight rating is considerably greater than the weight they'll be carrying. If that rating is less than they'll be carrying, replace with better.
Pull apart wheel
bearings and brakes. If brakes are disc, clean/grease caliper pins/slides. Check pads for adequate thickness.
If drums, check shoes, check/clean drums, make sure wheel cylinders are not corroded/stuck. Replace any parts
that don't look good. If shoes are greasy replace. Clean adjusters and lube their threads with anti-seize.
Clean bearings and races (and all other greasy parts) with solvent. Check for pitted or discolored surfaces. Replace bearings and races unless they look perfect. Unless you know they have few miles on them, you might want to replace anyway, since you have them apart.
Repack with high-quality marine
wheel bearing grease. Fill not just the bearings, but also the empty space around the spindle between the inner and outer bearings. Air left inside can expand when it gets warmer. Then when you launch, cool water
can suddenly contract
that air, and suck water
Tighten the castle nut until more than finger tight, then back off 1/6 or less before inserting cotter pin. Wheel should turn smooth and easy, with very little play.
If lugs or lug nuts are rusty or hacked up, clean up or replace. I put anti-seize on the lugs to keep salt water
out of the threads.
Make sure brake fluid is full and clean. You may need to bleed air bubbles from the brake lines - certainly if you have replaced wheel cylinders or calipers.
Adjust brakes if drums, and test.
Travel a few miles without using the brakes much at first, then stop and feel the wheel hubs. If the bearings are OK, they should not be very warm. If they feel hot to the touch, or one side is much warmer than the other, something is wrong.
If bearings seem OK, test the brakes, stopping moderately at first, and then braking harder. Brakes should provide plenty of stopping power, and evenly - the trailer should not try to swing to one side or the other when you apply the brakes. If brakes don't stop well or evenly (assuming all the parts
are in good shape) you could have air in the brake lines, or maybe drum brakes need better adjusting. Brakes should release as soon as you get going again. If they stick and drag, they'll got hot quickly and cause a problem - stop and check to see they're not very hot.
If bearings run cool and brakes work well and evenly, they should be OK. Check the brake fluid reservoir again after a successful road test.
For a long trip, I'd carry grease, solvent, tools, and spare bearings. And a jack and some pieces of wood.
Hope I didn't forget anything too important....