Dual antenna system covering HF to 70cm. I will being running a SBB-5 to support my Kenwood D710 and a an ATAS-120A to support the Yaesu 857D.
If you haven't purchased a truck cap yet, look at the various options.
Some are bare fiberglass on the inside. This might make it easier for installing a ground plane. Others have a carpeted interior, which may make installation a bit harder. Carpeted interiors are nice if you ever camp out of the back of your truck. Being inside moisture will build up on the interior and can condense and drip down on you. Carpet lined interiors don't suffer from this problem as much.
For the 2/70 antenna, you only need a 1/4 wave ground plane. You can accomplish this pretty easy with adhesive backed foil tape if you have the bare interior, but the adhesive may fail over time. Sheet metal, thin aluminum, window screen (as mentioned above) are all good options.
For HF, you are going to need to ground to the truck body, so figure on some braid running down to the truck bed, probably in all 4 corners.
If two antennas are all you plan on installing, then I'd give serious though to installing the 2/70 on the center of the cab roof. NMO mount permanently installed will work very well.
As for the HF, mounting an antenna that big to fiberglass is going to take some work. I'd be concerned about stress fractures in the gel coat over time. One tree branch hit will probably do some serious damage.
I think I'd go with a mount off the side of the truck bed, bumper, grill guard (like the Australians do), or something similar.